Pension and Other Postretirement Benefits Benefit Obligation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Award vesting period	3 years
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	$ 447,258
Balance, end of year	507,287	$ 447,258
Paid from Company assets for unfunded nonqualified benefits	1,634	1,537
Pension Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of period	634,068	612,688
Service cost	25,369	25,607	$ 22,205
Interest cost	28,263	25,203	26,817
Actuarial loss (gain)	35,504	21,731
Benefits paid from plan assets	(50,422)	(48,471)
Benefits paid from Company assets	(1,634)	(1,537)
Plan participants' contributions	0	0
Administrative expenses paid	(1,359)	(1,153)
Benefit obligation, end of period	669,789	634,068	612,688
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	447,258	470,165
Actual return on plan assets	31,810	(13,283)
Employer contributions	81,634	41,537
Plan participants' contributions	0	0
Benefits paid	(52,056)	(50,008)
Administrative expenses paid	(1,359)	(1,153)
Balance, end of year	507,287	447,258	470,165
Unfunded status	162,502	186,810
Accumulated benefit obligation	592,810	551,662
Pension and Other Postretirement Defined Benefit Plans, Liabilities [Abstract]
Current accrued benefit cost	1,552	1,599
Non-current accrued benefit cost	160,950	185,211
Net amount recognized	162,502	186,810
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), after Tax [Abstract]
Net actuarial loss	177,234	170,180
Prior service cost (gain)	457	716
Accumulated other comprehensive loss	177,691	$ 170,896
Pension and Other Postretirement Benefit Plans, Amounts that Will be Amortized from Accumulated Other Comprehensive Income (Loss) in Next Fiscal Year [Abstract]
Net actuarial loss	12,911
Prior service cost (gain)	257
Accumulated other comprehensive loss	$ 13,168
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Discount rate	4.15%	4.48%
Rate of compensation increase	4.52%	4.44%
Postretirement Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of period	$ 75,937	$ 77,355
Service cost	2,288	2,505	2,454
Interest cost	3,030	2,925	3,338
Actuarial loss (gain)	889	(2,165)
Benefits paid from plan assets	0	0
Benefits paid from Company assets	(6,958)	(6,131)
Plan participants' contributions	1,390	1,448
Administrative expenses paid	0	0
Benefit obligation, end of period	76,576	75,937	77,355
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	0	0
Actual return on plan assets	0	0
Employer contributions	5,568	4,683
Plan participants' contributions	1,390	1,448
Benefits paid	(6,958)	(6,131)
Administrative expenses paid	0	0
Balance, end of year	0	0	$ 0
Unfunded status	76,576	75,937
Pension and Other Postretirement Defined Benefit Plans, Liabilities [Abstract]
Current accrued benefit cost	5,948	5,634
Non-current accrued benefit cost	70,628	70,303
Net amount recognized	76,576	75,937
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), after Tax [Abstract]
Net actuarial loss	11,093	10,983
Prior service cost (gain)	(2,527)	(3,860)
Accumulated other comprehensive loss	8,566	$ 7,123
Pension and Other Postretirement Benefit Plans, Amounts that Will be Amortized from Accumulated Other Comprehensive Income (Loss) in Next Fiscal Year [Abstract]
Net actuarial loss	530
Prior service cost (gain)	(1,332)
Accumulated other comprehensive loss	$ (802)
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Discount rate	3.78%	4.09%
Rate of compensation increase	0.00%	0.00%
Health care cost trend rate, actuarial present value	6.00%	6.50%
Ultimate health care cost trend rate, actuarial present value	5.00%	5.00%
Health care cost trend rate, net periodic benefit cost	6.50%	7.00%	7.50%
Ultimate health care cost trend rate, net periodic benefit cost	5.00%	5.00%	5.00%
Year that rate reaches ultimate trend rate	2019