Pension Benefits (Details) - USD ($)		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Projected benefit obligation at beginning of year		$ 69,659,000	$ 62,547,000
Defined Benefit Plan, Service Cost		6,638,000	8,200,000	$ 7,949,000
Defined Benefit Plan, Interest Cost		2,689,000	2,908,000	2,496,000
Defined Benefit Plan, Actuarial Loss		3,708,000	2,662,000
Defined Benefit Plan, Benefits Paid		(10,757,000)	(6,658,000)
Projected benefit obligation at end of year		71,937,000	69,659,000	62,547,000
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Defined Benefit Plan, Fair value of plan assets at beginning of year		31,731,000	25,769,000
Defined Benefit Plan, Actual Return on Plan Assets		2,956,000	1,575,000
Defined Benefit Plan, Employer Contribution		7,048,000	11,045,000	6,400,000
Defined Benefit Plan, Benefits Paid		(10,757,000)	(6,658,000)	(8,200,000)
Defined Benefit Plan, Fair value of plan assets at end of year		30,978,000	31,731,000	25,769,000
Defined Benefit Plan, Funded (Unfunded) Status		(40,959,000)	(37,928,000)
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year		4,000,000
Defined Benefit Plan, Pension Plan with Accumulated Benefit Obligation in Excess of Plan Assets [Abstract]
Defined Benefit Plan, Projected Benefit Obligation		71,937,000	69,659,000
Defined Benefit Plan, Accumulated Benefit Obligation		56,045,000	54,681,000
Defined Benefit Plan, Fair Value of Plan Assets		30,978,000	31,731,000
Defined Benefit Plan, Unfunded Accumulated Benefit Obligation		$ 25,067,000	22,950,000
Defined Benefit Plan, Unrecognized Net Gain (Loss) Amortization Threshold		10.00%
Pre-tax Amounts Recognized in Accumulated Other Comprehensive Income [Abstract]
Defined Benefit Plan, Unrecognized Actuarial Losses		$ 21,397,000	22,708,000
Defined Benefit Plan, Unrecognized Prior Service Costs		66,000	83,000
Defined Benefit Plan, Accumulated Other Comprehensive Loss		21,463,000	22,791,000
Defined Benefit Plans, Amounts that Will be Amortized from Accumulated Other Comprehensive Loss) in Next Fiscal Year		1,300,000
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss) [Abstract]
Other Comprehensive Income (Loss), Defined Benefit Plan, Net Actuarial Loss		(2,995,000)	(3,322,000)	(4,990,000)
Other Comprehensive (Income) Loss, Defined Benefit Plan, Amortization of Prior Service Cost		17,000	16,000	17,000
Other Comprehensive Income (Loss), Defined Benefit Plan, Amortization of Net Actuarial Loss		1,297,000	1,582,000	1,486,000
Other Comprehensive Income (Loss), Defined Benefit Plan, Settlements		3,009,000	0	350,000
Other Comprehensive (Income) Loss, Defined Benefit Plan, Total Pension Liability Adjustment, Pre-Tax		1,328,000	(1,724,000)	(3,137,000)
Other Comprehensive (Income) Loss, Defined Benefit Plan, Tax (Expense) Benefit		(561,000)	570,000	1,047,000
Other Comprehensive (Income) Loss, Defined Benefit Plan, Total Pension Liability Adjustment, Net of Tax	[1]	(767,000)	1,154,000	2,090,000
Components of Net Periodic Benefit Costs for Both Pension Plans
Defined Benefit Plan, Service Cost		6,638,000	8,200,000	7,949,000
Defined Benefit Plan, Interest Cost		2,689,000	2,908,000	2,496,000
Defined Benefit Plan, Expected Return on Plan Assets that Reduces Periodic Pension Benefit Cost		(2,244,000)	(2,235,000)	(2,182,000)
Defined Benefit Plan, Amortization of Prior Service Cost		17,000	16,000	17,000
Defined Benefit Plan, Amortization Net Actuarial Loss		1,297,000	1,582,000	1,486,000
Defined Benefit Plan, Settlements		3,009,000	0	350,000
Defined Benefit Plan, Net Periodic Benefit Cost		$ 11,406,000	$ 10,471,000	$ 10,116,000
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation and Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate		3.80%	4.20%
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase		6.20%	6.20%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate		4.20%	4.70%	4.30%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Return on Plan Assets	[2]	6.50%	7.50%	7.50%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase		6.20%	6.20%	6.20%
Future Benefit Payments
Defined Benefit Plan, Expected Future Benefit Payments in Year One		$ 4,217,000
Defined Benefit Plan, Expected Future Benefit Payments in Year Two		3,818,000
Defined Benefit Plan, Expected Future Benefit Payments in Year Three		4,363,000
Defined Benefit Plan, Expected Future Benefit Payments in Year Four		5,561,000
Defined Benefit Plan, Expected Future Benefit Payments in Year Five		6,117,000
Defined Benefit Plan, Expected Future Benefit Payments in Five Fiscal Years Thereafter		36,279,000
Qualified Plan [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Defined Benefit Plan, Plan Assets Returned		0
Nonqualified Plan [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Defined Benefit Plan, Fair value of plan assets at beginning of year		0	$ 0
Defined Benefit Plan, Fair value of plan assets at end of year		$ 0	$ 0	$ 0
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation and Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Return on Plan Assets			0.00%	0.00%

[1] Please refer to Note 8 – Pension Benefits for additional discussion on the pension liability adjustment.

[2] There is no assumed expected return on plan assets for the Nonqualified Pension Plan because there are no plan assets in the Nonqualified Pension Plan.