Employee Benefit Plans Investment Strategy (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Target asset allocation allowable range of plus or minus	5.00%
Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	100.00%		100.00%
Cash and Cash Equivalents [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	0.20%		1.80%
Debt Securities [Member] | Pension Plan [Member] | Domestic [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Target allocation of investments by plan	55.00%		60.00%
Debt Securities [Member] | Pension Plan [Member] | International [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Target allocation of investments by plan	5.00%		5.00%
Debt Securities [Member] | Other Postretirement Benefit Plan [Member] | Domestic [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Target allocation of investments by plan	40.00%		40.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	37.20%		38.60%
Debt Securities [Member] | Other Postretirement Benefit Plan [Member] | International [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Target allocation of investments by plan	0.00%		0.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	0.00%		0.30%
Equity Securities [Member] | Pension Plan [Member] | Domestic [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Target allocation of investments by plan	34.00%		30.00%
Equity Securities [Member] | Pension Plan [Member] | International [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Target allocation of investments by plan	6.00%		5.00%
Equity Securities [Member] | Other Postretirement Benefit Plan [Member] | Domestic [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Target allocation of investments by plan	50.00%		50.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	53.90%		50.10%
Equity Securities [Member] | Other Postretirement Benefit Plan [Member] | International [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Target allocation of investments by plan	10.00%		10.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	8.70%		9.20%
SOUTH DAKOTA | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	100.00%		100.00%
SOUTH DAKOTA | Cash and Cash Equivalents [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	0.10%		0.10%
SOUTH DAKOTA | Debt Securities [Member] | Pension Plan [Member] | Domestic [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	65.60%		64.70%
SOUTH DAKOTA | Debt Securities [Member] | Pension Plan [Member] | International [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	4.50%		4.90%
SOUTH DAKOTA | Equity Securities [Member] | Pension Plan [Member] | Domestic [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	25.10%		25.30%
SOUTH DAKOTA | Equity Securities [Member] | Pension Plan [Member] | International [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	4.70%		5.00%
MONTANA | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	100.00%		100.00%
MONTANA | Cash and Cash Equivalents [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	0.00%		0.00%
MONTANA | Debt Securities [Member] | Pension Plan [Member] | Domestic [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	56.00%		58.60%
MONTANA | Debt Securities [Member] | Pension Plan [Member] | International [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	4.40%		4.90%
MONTANA | Equity Securities [Member] | Pension Plan [Member] | Domestic [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	34.10%		31.40%
MONTANA | Equity Securities [Member] | Pension Plan [Member] | International [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	5.50%		5.10%
Fair Value, Inputs, Level 1 [Member] | Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Cash and cash equivalents	0		0
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0		0
Fair Value, Inputs, Level 1 [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Cash and cash equivalents	0		0
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0		0
Fair Value, Inputs, Level 1 [Member] | US Small Midcap Growth [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 1 [Member] | US Small Midcap Growth [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 1 [Member] | US Small Midcap Value [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 1 [Member] | US Small Midcap Value [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 1 [Member] | S&P 500 Index [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 1 [Member] | US Large Cap Growth [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 1 [Member] | US Large Cap Growth [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 1 [Member] | US Large Cap Value [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 1 [Member] | US Large Cap Value [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 1 [Member] | US Large Cap Passive [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 1 [Member] | US Large Cap Passive [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 1 [Member] | Non US Core [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 1 [Member] | Non US Core [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 1 [Member] | Emerging Markets [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 1 [Member] | Emerging Markets [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 1 [Member] | Passive Bond Market [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 1 [Member] | US Core Opportunistic [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 1 [Member] | US Core Opportunistic [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 1 [Member] | US Passive [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 1 [Member] | US Passive [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0		0
Fair Value, Inputs, Level 1 [Member] | Long Duration [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 1 [Member] | Long Duration [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 1 [Member] | Long Duration Investment Grade [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0		0
Fair Value, Inputs, Level 1 [Member] | Long Duration Investment Grade [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 1 [Member] | Long Duration Passive [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 1 [Member] | Long Duration Passive [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0		0
Fair Value, Inputs, Level 1 [Member] | Opportunistic [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 1 [Member] | Opportunistic [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0		0
Fair Value, Inputs, Level 1 [Member] | Non US Passive [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 1 [Member] | Non US Passive [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 1 [Member] | Active Long Corporate [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 1 [Member] | Active Long Corporate [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0		0
Fair Value, Inputs, Level 1 [Member] | Participating Group Annuity Contract [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 2 [Member] | Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Cash and cash equivalents	126		168
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	556,051		516,352
Fair Value, Inputs, Level 2 [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Cash and cash equivalents	44		318
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	18,040		18,183
Fair Value, Inputs, Level 2 [Member] | US Small Midcap Growth [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	16,605	[1]	13,764
Fair Value, Inputs, Level 2 [Member] | US Small Midcap Growth [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	752	[1]	751
Fair Value, Inputs, Level 2 [Member] | US Small Midcap Value [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	15,264	[1]	13,664
Fair Value, Inputs, Level 2 [Member] | US Small Midcap Value [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	721	[1]	736
Fair Value, Inputs, Level 2 [Member] | S&P 500 Index [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	8,234	[1]	7,321
Fair Value, Inputs, Level 2 [Member] | US Large Cap Growth [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	48,560	[1]	42,094
Fair Value, Inputs, Level 2 [Member] | US Large Cap Growth [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	6	[1]	98
Fair Value, Inputs, Level 2 [Member] | US Large Cap Value [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	48,785	[1]	42,102
Fair Value, Inputs, Level 2 [Member] | US Large Cap Value [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	6	[1]	98
Fair Value, Inputs, Level 2 [Member] | US Large Cap Passive [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	54,775	[1]	47,227
Fair Value, Inputs, Level 2 [Member] | US Large Cap Passive [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	7	[1]	110
Fair Value, Inputs, Level 2 [Member] | Non US Core [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	22,634	[1]	20,015
Fair Value, Inputs, Level 2 [Member] | Non US Core [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	1,495	[1]	1,595
Fair Value, Inputs, Level 2 [Member] | Emerging Markets [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	7,650	[1]	6,250
Fair Value, Inputs, Level 2 [Member] | Emerging Markets [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	72	[1]	85
Fair Value, Inputs, Level 2 [Member] | Passive Bond Market [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	1,992	[2]	1,880
Fair Value, Inputs, Level 2 [Member] | US Core Opportunistic [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	23,177	[2]	82,639
Fair Value, Inputs, Level 2 [Member] | US Core Opportunistic [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	4,435	[2]	4,390
Fair Value, Inputs, Level 2 [Member] | US Passive [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	12,269	[2]	44,762
Fair Value, Inputs, Level 2 [Member] | US Passive [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	1	[2]	107
Fair Value, Inputs, Level 2 [Member] | Long Duration [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	41,451	[2]	24,401
Fair Value, Inputs, Level 2 [Member] | Long Duration [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	5	[2]	55
Fair Value, Inputs, Level 2 [Member] | Long Duration Investment Grade [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	52,767		32,700
Fair Value, Inputs, Level 2 [Member] | Long Duration Investment Grade [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	6	[2]	79
Fair Value, Inputs, Level 2 [Member] | Long Duration Passive [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	41,475		24,122
Fair Value, Inputs, Level 2 [Member] | Long Duration Passive [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	5	[2]	55
Fair Value, Inputs, Level 2 [Member] | Opportunistic [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	5,692		5,876
Fair Value, Inputs, Level 2 [Member] | Opportunistic [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	240	[2]	261
Fair Value, Inputs, Level 2 [Member] | Non US Passive [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	24,504	[2]	25,150
Fair Value, Inputs, Level 2 [Member] | Non US Passive [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	3	[2]	57
Fair Value, Inputs, Level 2 [Member] | Active Long Corporate [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	133,160	[2]	83,147
Fair Value, Inputs, Level 2 [Member] | Active Long Corporate [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	16	[2]	187
Fair Value, Inputs, Level 2 [Member] | Participating Group Annuity Contract [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	7,157	[2]	8,271
Fair Value, Inputs, Level 3 [Member] | Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Cash and cash equivalents	0		0
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0		0
Fair Value, Inputs, Level 3 [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Cash and cash equivalents	0		0
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0		0
Fair Value, Inputs, Level 3 [Member] | US Small Midcap Growth [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 3 [Member] | US Small Midcap Growth [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 3 [Member] | US Small Midcap Value [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 3 [Member] | US Small Midcap Value [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 3 [Member] | S&P 500 Index [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 3 [Member] | US Large Cap Growth [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 3 [Member] | US Large Cap Growth [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 3 [Member] | US Large Cap Value [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 3 [Member] | US Large Cap Value [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 3 [Member] | US Large Cap Passive [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 3 [Member] | US Large Cap Passive [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 3 [Member] | Non US Core [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 3 [Member] | Non US Core [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 3 [Member] | Emerging Markets [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 3 [Member] | Emerging Markets [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[1]	0
Fair Value, Inputs, Level 3 [Member] | Passive Bond Market [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 3 [Member] | US Core Opportunistic [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 3 [Member] | US Core Opportunistic [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 3 [Member] | US Passive [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 3 [Member] | US Passive [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 3 [Member] | Long Duration [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 3 [Member] | Long Duration [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 3 [Member] | Long Duration Investment Grade [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 3 [Member] | Long Duration Investment Grade [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0		0
Fair Value, Inputs, Level 3 [Member] | Long Duration Passive [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 3 [Member] | Long Duration Passive [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0		0
Fair Value, Inputs, Level 3 [Member] | Opportunistic [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 3 [Member] | Opportunistic [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0		0
Fair Value, Inputs, Level 3 [Member] | Non US Passive [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0		0
Fair Value, Inputs, Level 3 [Member] | Non US Passive [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0		0
Fair Value, Inputs, Level 3 [Member] | Active Long Corporate [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Fair Value, Inputs, Level 3 [Member] | Active Long Corporate [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0		0
Fair Value, Inputs, Level 3 [Member] | Participating Group Annuity Contract [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	0	[2]	0
Estimate of Fair Value Measurement [Member] | Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Cash and cash equivalents	126		168
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	556,051		516,352
Estimate of Fair Value Measurement [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Cash and cash equivalents	44		318
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	18,040		18,183
Estimate of Fair Value Measurement [Member] | US Small Midcap Growth [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	16,605	[1]	13,764	[1]
Estimate of Fair Value Measurement [Member] | US Small Midcap Growth [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	752	[1]	751	[1]
Estimate of Fair Value Measurement [Member] | US Small Midcap Value [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	15,264	[1]	13,664	[1]
Estimate of Fair Value Measurement [Member] | US Small Midcap Value [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	721	[1]	736	[1]
Estimate of Fair Value Measurement [Member] | S&P 500 Index [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	8,234	[1]	7,321	[1]
Estimate of Fair Value Measurement [Member] | US Large Cap Growth [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	48,560	[1]	42,094	[1]
Estimate of Fair Value Measurement [Member] | US Large Cap Growth [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	6	[1]	98	[1]
Estimate of Fair Value Measurement [Member] | US Large Cap Value [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	48,785	[1]	42,102	[1]
Estimate of Fair Value Measurement [Member] | US Large Cap Value [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	6	[1]	98	[1]
Estimate of Fair Value Measurement [Member] | US Large Cap Passive [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	54,775	[1]	47,227	[1]
Estimate of Fair Value Measurement [Member] | US Large Cap Passive [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	7	[1]	110	[1]
Estimate of Fair Value Measurement [Member] | Non US Core [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	22,634	[1]	20,015	[1]
Estimate of Fair Value Measurement [Member] | Non US Core [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	1,495	[1]	1,595	[1]
Estimate of Fair Value Measurement [Member] | Emerging Markets [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	7,650	[1]	6,250
Estimate of Fair Value Measurement [Member] | Emerging Markets [Member] | Equity Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	72	[1]	85
Estimate of Fair Value Measurement [Member] | Passive Bond Market [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	1,992	[2]	1,880	[2]
Estimate of Fair Value Measurement [Member] | US Core Opportunistic [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	23,177	[2]	82,639	[2]
Estimate of Fair Value Measurement [Member] | US Core Opportunistic [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	4,435	[2]	4,390	[2]
Estimate of Fair Value Measurement [Member] | US Passive [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	12,269	[2]	44,762	[2]
Estimate of Fair Value Measurement [Member] | US Passive [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	1	[2]	107	[2]
Estimate of Fair Value Measurement [Member] | Long Duration [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	41,451	[2]	24,401	[2]
Estimate of Fair Value Measurement [Member] | Long Duration [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	5	[2]	55	[2]
Estimate of Fair Value Measurement [Member] | Long Duration Investment Grade [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	52,767	[2]	32,700	[2]
Estimate of Fair Value Measurement [Member] | Long Duration Investment Grade [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	6	[2]	79	[2]
Estimate of Fair Value Measurement [Member] | Long Duration Passive [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	41,475	[2]	24,122	[2]
Estimate of Fair Value Measurement [Member] | Long Duration Passive [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	5	[2]	55	[2]
Estimate of Fair Value Measurement [Member] | Opportunistic [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	5,692	[2]	5,876	[2]
Estimate of Fair Value Measurement [Member] | Opportunistic [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	240	[2]	261	[2]
Estimate of Fair Value Measurement [Member] | Non US Passive [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	24,504	[2]	25,150	[2]
Estimate of Fair Value Measurement [Member] | Non US Passive [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	3	[2]	57	[2]
Estimate of Fair Value Measurement [Member] | Active Long Corporate [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	133,160	[2]	83,147	[2]
Estimate of Fair Value Measurement [Member] | Active Long Corporate [Member] | Debt Securities [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	16	[2]	187	[2]
Estimate of Fair Value Measurement [Member] | Participating Group Annuity Contract [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Available-for-sale Securities	7,157	[2]	8,271	[2]

[1] This category consists of active and passive managed equity funds, which are invested in multiple strategies to diversify risks and reduce volatility.

[2] This category consists of investment grade bonds of issuers from diverse industries, debt securities issued by international, national, state and local governments, and asset-backed securities. This includes both active and passive managed funds.