Pension and Other Postretirement Benefits (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]
Defined benefit plan fair value of plan assets measured at net asset value	$ 351,483	$ 172,521
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Estimated nonqualified benefits in next fiscal year	1,626
Estimated future employer contributions in next fiscal year	$ 40,000
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Actual Allocation	100.00%	100.00%
Defined Benefit Plan, Target Allocation Percentage	1.0	1.0
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
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	$ 11,488	$ 9,827
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Actual Allocation	8.00%	8.00%
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	$ 40,779
Balance, end of year	46,438	$ 40,779
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	10.00%	11.00%
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	$ 55,619
Balance, end of year	59,632	$ 55,619
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	$ 296,232	$ 99,383
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Actual Allocation	70.00%	62.00%
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	$ 318,095
Balance, end of year	410,679	$ 318,095
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	3.00%	5.00%
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	$ 27,346
Balance, end of year	17,210	$ 27,346
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	$ 24,444	$ 25,479
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Actual Allocation	4.00%	5.00%
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	$ 25,479
Balance, end of year	24,444	$ 25,479
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	$ 15,565	$ 14,852
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Actual Allocation	3.00%	3.00%
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	$ 15,047
Balance, end of year	15,762	$ 15,047
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%
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	$ 1,942
Balance, end of year	7,461	$ 1,942
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	$ 3,754	$ 22,980
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Actual Allocation	1.00%	5.00%
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	$ 22,980
Balance, end of year	3,754	$ 22,980
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	332,629
Balance, end of year	226,239	332,629
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	30,952
Balance, end of year	34,950	30,952
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	55,619
Balance, end of year	59,632	55,619
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	218,712
Balance, end of year	114,447	218,712
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	27,346
Balance, end of year	17,210	27,346
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	2,137
Balance, end of year	7,658	2,137
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	195
Balance, end of year	197	195
Significant Unobservable Inputs (Level 3) | Private equity
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	1,942
Balance, end of year	7,461	1,942
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 (Credit) [Abstract]
Service cost	26,415	25,369	$ 25,607
Interest cost	27,818	28,263	25,203
Expected return on plan assets	(30,631)	(27,016)	(28,297)
Net actuarial loss	21,503	19,164	20,028
Prior service cost (gain)	420	423	452
Net periodic benefit cost	$ 45,525	$ 46,203	$ 42,993
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Discount rate	4.15%	4.48%	4.08%
Expected return on plan assets	5.75%	5.75%	6.25%
Rate of compensation increase	4.52%	4.44%	4.47%
Defined Benefit Plan, Expected Future Benefit Payment [Abstract]
2018	$ 48,665
2019	51,270
2020	52,701
2021	51,088
2022	52,146
After 2022	276,523
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	507,287	$ 447,258
Realized gains/(losses)	69,832	31,810
Balance, end of year	585,380	507,287	$ 447,258
Pension Benefits | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	2,137	3,160
Realized gains/(losses)	96	193
Unrealized gains/(losses)	(98)	(169)
Purchases	6,891	121
Sales	(1,368)	(1,168)
Balance, end of year	7,658	2,137	3,160
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	195	456
Realized gains/(losses)	0	12
Unrealized gains/(losses)	2	(19)
Purchases	0	0
Sales	0	(254)
Balance, end of year	197	195	456
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	1,942	2,704
Realized gains/(losses)	96	181
Unrealized gains/(losses)	(100)	(150)
Purchases	6,891	121
Sales	(1,368)	(914)
Balance, end of year	7,461	1,942	2,704
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	2,327	2,288	2,505
Interest cost	2,844	3,030	2,925
Expected return on plan assets	0	0	0
Net actuarial loss	788	709	1,044
Prior service cost (gain)	(2,181)	(2,181)	(2,181)
Net periodic benefit cost	$ 3,778	$ 3,846	$ 4,293
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Discount rate	3.78%	4.09%	3.77%
Expected return on plan assets	0.00%	0.00%	0.00%
Rate of compensation increase	0.00%	0.00%	0.00%
Ultimate health care cost trend rate	5.00%
Defined Benefit Plan, Expected Future Benefit Payment [Abstract]
2018	$ 6,156
2019	6,565
2020	6,856
2021	7,026
2022	7,003
After 2022	33,742
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
Minimum | Equity Securities - U.S.
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan, Target Allocation Percentage	0.03	.03
Minimum | Equity Securities - International
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan, Target Allocation Percentage	.03	.03
Minimum | Fixed Income Investments - U.S.
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan, Target Allocation Percentage	.35	.35
Minimum | Fixed Income Investments - International
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan, Target Allocation Percentage	.03	.03
Minimum | Absolute Return
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan, Target Allocation Percentage	.05	.05
Minimum | Real assets
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan, Target Allocation Percentage	.03	.03
Minimum | Private equity
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan, Target Allocation Percentage	0	0
Minimum | Short-term Investments
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan, Target Allocation Percentage	.0	0
Maximum | Equity Securities - U.S.
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan, Target Allocation Percentage	.15	.15
Maximum | Equity Securities - International
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan, Target Allocation Percentage	.15	.15
Maximum | Fixed Income Investments - U.S.
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan, Target Allocation Percentage	.75	.75
Maximum | Fixed Income Investments - International
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan, Target Allocation Percentage	.10	.10
Maximum | Absolute Return
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan, Target Allocation Percentage	.15	.15
Maximum | Real assets
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan, Target Allocation Percentage	.10	.10
Maximum | Private equity
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan, Target Allocation Percentage	.03	.03
Maximum | Short-term Investments
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan, Target Allocation Percentage	.03	.03