Pension and OPEB Pension Plan Assets Fair Value (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost	$ 22,196	$ 19,053	$ 29,095
Defined Benefit Plan, Fair Value of Plan Assets	632,434	669,039
Defined Benefit Plan, Actuarial Net (Gains) Losses	(23,857)	(119,222)	(46,155)
Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	650	641
Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0	0
Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0	0
Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	639	631
Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	639	631
Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0	0
Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0	0
Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	11	10
Equity Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	11	10
Equity Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0	0
Equity Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0	0
Private Equity Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	631,784	668,398
Private Equity Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0	0
Private Equity Funds [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0	0
Private Equity Funds [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0	0
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Benefit Obligation	747,881	763,407	870,471
Defined Benefit Plan, Net Periodic Benefit Cost	22,196	19,053	29,095
Defined Benefit Plan, Fair Value of Plan Assets	632,434	669,039	751,176
Defined Benefit Plan, Service Cost	1,927	8,653	17,187
service cost - prior year		8,653
Defined Benefit Plan, Interest Cost	25,491	32,095	35,363
Defined Benefit Plan, Actuarial Net (Gains) Losses	46,962	(39,563)
Defined Benefit Plan, Plan Amendments	0	0
Defined Benefit Plan, Curtailments	0	(26,352)	(21,624)
Defined Benefit Plan, Settlements, Benefit Obligation	(54,197)	(51,497)
Defined Benefit Plan, Contributions by Plan Participants	0	0
Defined Benefit Plan, Benefits Paid	35,709	30,400
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Benefit Obligation	700,085	671,755	760,959
Defined Benefit Plan, Net Periodic Benefit Cost	0	(8,932)	0
Defined Benefit Plan, Fair Value of Plan Assets	0	0	0
Defined Benefit Plan, Service Cost	0	0	7,089
Defined Benefit Plan, Interest Cost	24,241	27,238	44,177
Defined Benefit Plan, Actuarial Net (Gains) Losses	77,640	(9,224)
Defined Benefit Plan, Plan Amendments	(28,164)	43,598	$ 315,439
Defined Benefit Plan, Curtailments	0	0
Defined Benefit Plan, Settlements, Benefit Obligation	0	0
Defined Benefit Plan, Contributions by Plan Participants	0	1,649
Defined Benefit Plan, Benefits Paid	$ 45,387	$ 60,027