Pension and Other Postretirement Benefits Benefit Obligation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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	$ 507,287
Balance, end of year	585,380	$ 507,287
Paid from Company assets for unfunded nonqualified benefits	1,589	1,634
Pension Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of period	669,789	634,068
Service cost	26,415	25,369	$ 25,607
Interest cost	27,818	28,263	25,203
Actuarial loss (gain)	85,618	35,504
Benefits paid from plan assets	(50,136)	(50,422)
Benefits paid from Company assets	(1,589)	(1,634)
Plan participants' contributions	0	0
Administrative expenses paid	(1,603)	(1,359)
Benefit obligation, end of period	756,312	669,789	634,068
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	507,287	447,258
Actual return on plan assets	69,832	31,810
Employer contributions	61,589	81,634
Defined Benefit Plan, Plan Assets, Contributions by Plan Participant	0	0
Benefits paid	(51,725)	(52,056)
Administrative expenses paid	(1,603)	(1,359)
Balance, end of year	585,380	507,287	$ 447,258
Unfunded status	170,932	162,502
Accumulated benefit obligation	670,060	592,810
Liability, Defined Benefit Plan [Abstract]
Current accrued benefit cost	1,707	1,552
Non-current accrued benefit cost	169,225	160,950
Net amount recognized	170,932	162,502
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), after Tax [Abstract]
Net actuarial loss	192,456	177,234
Prior service cost (gain)	201	457
Accumulated other comprehensive loss	192,657	$ 177,691
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	19,965
Prior service cost (gain)	235
Accumulated other comprehensive loss	$ 20,200
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Discount rate	3.67%	4.15%
Rate of compensation increase	4.74%	4.52%
Postretirement Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Health Care Cost Trend Rate Assumed for Next Fiscal Year, Net Periodic Benefit Cost	6.00%	6.50%	7.00%
Defined Benefit Plan, Health Care Cost Trend Rate Assumed, for Next Fiscal Year, Actuarial Present Value	5.50%	6.50%
Defined Benefit Plan, Ultimate Health Care Cost Trend Rate, Actuarial Present Value	5.00%	5.00%
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of period	$ 76,576	$ 75,937
Service cost	2,327	2,288	$ 2,505
Interest cost	2,844	3,030	2,925
Actuarial loss (gain)	(61)	889
Benefits paid from plan assets	0	0
Benefits paid from Company assets	(6,427)	(6,958)
Plan participants' contributions	1,540	1,390
Administrative expenses paid	0	0
Benefit obligation, end of period	76,799	76,576	75,937
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	4,887	5,568
Defined Benefit Plan, Plan Assets, Contributions by Plan Participant	1,540	1,390
Benefits paid	(6,427)	(6,958)
Administrative expenses paid	0	0
Balance, end of year	0	0	$ 0
Unfunded status	76,799	76,576
Liability, Defined Benefit Plan [Abstract]
Current accrued benefit cost	6,052	5,948
Non-current accrued benefit cost	70,747	70,628
Net amount recognized	76,799	76,576
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), after Tax [Abstract]
Net actuarial loss	10,575	11,093
Prior service cost (gain)	(1,195)	(2,527)
Accumulated other comprehensive loss	9,380	$ 8,566
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	613
Prior service cost (gain)	(1,453)
Accumulated other comprehensive loss	$ (840)
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Discount rate	3.44%	3.78%
Rate of compensation increase	0.00%	0.00%
Defined Benefit Plan Ultimate Health Care Cost Trend Rate For Benefit Obligation, Net Periodic Benefit Cost	5.00%	5.00%	5.00%