Pension and Other Postretirement Benefits (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]
Defined benefit plan fair value of plan assets measured at net asset value	$ 172,521	$ 130,268
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Health care cost trend rate assumed for next fiscal year	6.50%
Ultimate health care cost trend rate	5.00%
Estimated nonqualified benefits in next fiscal year	$ 1,589
Estimated future employer contributions in next fiscal year	$ 60,000
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Actual Allocation	100.00%	100.00%
Target allocation range, maximum	100.00%	100.00%
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
Equity Securities - U.S.
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan fair value of plan assets measured at net asset value	$ 9,827	$ 9,206
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Actual Allocation	8.00%	12.00%
Target allocation range, minimum	3.00%	3.00%
Target allocation range, maximum	15.00%	15.00%
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	$ 52,766
Balance, end of year	40,779	$ 52,766
Equity Securities - International
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan fair value of plan assets measured at net asset value	$ 0	$ 0
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Actual Allocation	11.00%	11.00%
Target allocation range, minimum	3.00%	3.00%
Target allocation range, maximum	15.00%	15.00%
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	$ 48,004
Balance, end of year	55,619	$ 48,004
Fixed Income Investments - U.S.
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan fair value of plan assets measured at net asset value	$ 99,383	$ 70,949
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Actual Allocation	62.00%	59.00%
Target allocation range, minimum	35.00%	35.00%
Target allocation range, maximum	75.00%	75.00%
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	$ 267,171
Balance, end of year	318,095	$ 267,171
Fixed Income Investments - International
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan fair value of plan assets measured at net asset value	$ 0	$ 0
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Actual Allocation	5.00%	6.00%
Target allocation range, minimum	3.00%	3.00%
Target allocation range, maximum	10.00%	10.00%
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	$ 26,044
Balance, end of year	27,346	$ 26,044
Absolute Return
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan fair value of plan assets measured at net asset value	$ 25,479	$ 32,383
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Actual Allocation	5.00%	7.00%
Target allocation range, minimum	5.00%	5.00%
Target allocation range, maximum	15.00%	15.00%
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	$ 32,383
Balance, end of year	25,479	$ 32,383
Real assets
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan fair value of plan assets measured at net asset value	$ 14,852	$ 13,166
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Actual Allocation	3.00%	3.00%
Target allocation range, minimum	3.00%	3.00%
Target allocation range, maximum	10.00%	10.00%
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	$ 13,622
Balance, end of year	15,047	$ 13,622
Private equity
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan fair value of plan assets measured at net asset value	$ 0	$ 0
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Actual Allocation	1.00%	1.00%
Target allocation range, minimum	0.00%	0.00%
Target allocation range, maximum	3.00%	3.00%
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	$ 2,704
Balance, end of year	1,942	$ 2,704
Short-term Investments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan fair value of plan assets measured at net asset value	$ 22,980	$ 4,564
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Actual Allocation	5.00%	1.00%
Target allocation range, minimum	0.00%	0.00%
Target allocation range, maximum	3.00%	3.00%
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	$ 4,564
Balance, end of year	22,980	$ 4,564
Quoted Prices in Active Markets for Identical Inputs (Level 1)
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	313,830
Balance, end of year	332,629	313,830
Quoted Prices in Active Markets for Identical Inputs (Level 1) | Equity Securities - U.S.
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	43,560
Balance, end of year	30,952	43,560
Quoted Prices in Active Markets for Identical Inputs (Level 1) | Equity Securities - International
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	48,004
Balance, end of year	55,619	48,004
Quoted Prices in Active Markets for Identical Inputs (Level 1) | Fixed Income Investments - U.S.
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	196,222
Balance, end of year	218,712	196,222
Quoted Prices in Active Markets for Identical Inputs (Level 1) | Fixed Income Investments - International
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	26,044
Balance, end of year	27,346	26,044
Quoted Prices in Active Markets for Identical Inputs (Level 1) | Absolute Return
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	0
Balance, end of year	0	0
Quoted Prices in Active Markets for Identical Inputs (Level 1) | Real assets
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	0
Balance, end of year	0	0
Quoted Prices in Active Markets for Identical Inputs (Level 1) | Private equity
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	0
Balance, end of year	0	0
Quoted Prices in Active Markets for Identical Inputs (Level 1) | Short-term Investments
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	0
Balance, end of year	0	0
Significant Other Observable Inputs (Level 2)
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	0
Balance, end of year	0	0
Significant Other Observable Inputs (Level 2) | Equity Securities - U.S.
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	0
Balance, end of year	0	0
Significant Other Observable Inputs (Level 2) | Equity Securities - International
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	0
Balance, end of year	0	0
Significant Other Observable Inputs (Level 2) | Fixed Income Investments - U.S.
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	0
Balance, end of year	0	0
Significant Other Observable Inputs (Level 2) | Fixed Income Investments - International
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	0
Balance, end of year	0	0
Significant Other Observable Inputs (Level 2) | Absolute Return
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	0
Balance, end of year	0	0
Significant Other Observable Inputs (Level 2) | Real assets
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	0
Balance, end of year	0	0
Significant Other Observable Inputs (Level 2) | Private equity
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	0
Balance, end of year	0	0
Significant Other Observable Inputs (Level 2) | Short-term Investments
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	0
Balance, end of year	0	0
Significant Unobservable Inputs (Level 3)
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	3,160
Balance, end of year	2,137	3,160
Significant Unobservable Inputs (Level 3) | Equity Securities - U.S.
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	0
Balance, end of year	0	0
Significant Unobservable Inputs (Level 3) | Equity Securities - International
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	0
Balance, end of year	0	0
Significant Unobservable Inputs (Level 3) | Fixed Income Investments - U.S.
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	0
Balance, end of year	0	0
Significant Unobservable Inputs (Level 3) | Fixed Income Investments - International
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	0
Balance, end of year	0	0
Significant Unobservable Inputs (Level 3) | Absolute Return
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	0
Balance, end of year	0	0
Significant Unobservable Inputs (Level 3) | Real assets
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	456
Balance, end of year	195	456
Significant Unobservable Inputs (Level 3) | Private equity
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	2,704
Balance, end of year	1,942	2,704
Significant Unobservable Inputs (Level 3) | Short-term Investments
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	0
Balance, end of year	0	0
Pension Benefits
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost	25,369	25,607	$ 22,205
Interest cost	28,263	25,203	26,817
Expected return on plan assets	(27,016)	(28,297)	(26,624)
Net actuarial loss	19,164	20,028	17,639
Prior service cost (gain)	423	452	952
Settlement loss	0	0	789
Net periodic benefit cost	$ 46,203	$ 42,993	$ 41,778
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Discount rate	4.48%	4.08%	4.87%
Expected return on plan assets	5.75%	6.25%	6.25%
Rate of compensation increase	4.44%	4.47%	4.25%
Defined Benefit Plan, Expected Future Benefit Payments, Fiscal Year Maturity [Abstract]
2017	$ 42,035
2018	43,351
2019	45,689
2020	47,949
2021	46,678
After 2021	259,183
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	447,258	$ 470,165
Realized gains/(losses)	31,810	(13,283)
Balance, end of year	507,287	447,258	$ 470,165
Pension Benefits | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	3,160	4,230
Realized gains/(losses)	193	174
Unrealized gains/(losses)	(169)	(129)
Purchases	121	188
Sales	(1,168)	(1,303)
Balance, end of year	2,137	3,160	4,230
Pension Benefits | Significant Unobservable Inputs (Level 3) | Real assets
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	456	548
Realized gains/(losses)	12	0
Unrealized gains/(losses)	(19)	(37)
Purchases	0	0
Sales	(254)	(55)
Balance, end of year	195	456	548
Pension Benefits | Significant Unobservable Inputs (Level 3) | Private equity
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	2,704	3,682
Realized gains/(losses)	181	174
Unrealized gains/(losses)	(150)	(92)
Purchases	121	188
Sales	(914)	(1,248)
Balance, end of year	1,942	2,704	3,682
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost	2,288	2,505	2,454
Interest cost	3,030	2,925	3,338
Expected return on plan assets	0	0	0
Net actuarial loss	709	1,044	1,267
Prior service cost (gain)	(2,181)	(2,181)	(2,181)
Settlement loss	0	0	0
Net periodic benefit cost	$ 3,846	$ 4,293	$ 4,878
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Discount rate	4.09%	3.77%	4.34%
Expected return on plan assets	0.00%	0.00%	0.00%
Rate of compensation increase	0.00%	0.00%	0.00%
Health care cost trend rate assumed for next fiscal year	6.00%
Ultimate health care cost trend rate	5.00%
Year that rate reaches ultimate trend rate	2019
Defined Benefit Plan, Expected Future Benefit Payments, Fiscal Year Maturity [Abstract]
2017	$ 6,059
2018	6,449
2019	6,711
2020	6,893
2021	6,992
After 2021	33,882
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	0	$ 0
Realized gains/(losses)	0	0
Balance, end of year	$ 0	$ 0	$ 0