Pension Benefits (Details) - USD ($)		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Defined Benefit Plan, Benefit Obligation Beginning of Year		$ 62,547,000	$ 57,867,000
Defined Benefit Plan, Service Cost		8,200,000	7,949,000	$ 6,335,000
Defined Benefit Plan, Interest Cost		2,908,000	2,496,000	2,191,000
Defined Benefit Plan, Actuarial Gain (Loss)		2,662,000	2,397,000
Defined Benefit Plan, Benefits Paid		(6,658,000)	(8,162,000)	(2,800,000)
Defined Benefit Plan, Benefit Obligation End of Year		69,659,000	62,547,000	57,867,000
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Defined Benefit Plan, Fair Value of Plan Assets Beginning of Year		25,769,000	27,940,000
Defined Benefit Plan, Actual Return on Plan Assets		1,575,000	(410,000)
Defined Benefit Plan, Employer Contribution		11,045,000	6,401,000	5,300,000
Defined Benefit Plan, Fair Value of Plan Assets End of Year		31,731,000	25,769,000	27,940,000
Defined Benefit Plan, Funded Status of Plan		(37,928,000)	(36,778,000)
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year		7,800,000
Defined Benefit Plan, Pension Plans with Accumulated Benefit Obligations in Excess of Plan Assets [Abstract]
Defined Benefit Plan, Benefit Obligation End of Year		69,659,000	62,547,000	57,867,000
Defined Benefit Plan, Accumulated Benefit Obligation		54,681,000	46,439,000
Defined Benefit Plan, Fair Value of Plan Assets Beginning of Year		(31,731,000)	(25,769,000)	(27,940,000)
Defined Benefit Plan, Accumulated Unfunded Benefit Obligation		$ 22,950,000	20,670,000
Pension And Other Post-retirement Benefit Plans, Gain (Loss) Amortization Threshold		10.00%
Pre-tax Amounts Recognized in Accumulated Other Comprehensive Income
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Gains (Losses), before Tax		$ 22,708,000	20,966,000	17,812,000
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Prior Service Cost (Credit), before Tax		83,000	101,000	118,000
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Transition Assets (Obligations), before Tax		0	0	0
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), before Tax		22,791,000	21,067,000	17,930,000
Pension and Other Postretirement Benefit Plans, Amounts that Will be Amortized from Accumulated Other Comprehensive Income (Loss) in Next Fiscal Year		1,600,000
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss) [Abstract]
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Arising During Period, before Tax		(3,322,000)	(4,990,000)	(10,062,000)
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Prior Service Cost Arising During Period, before Tax		0	0	0
Other Comprehensive (Income) Loss, Amortization Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, for Net Prior Service Cost (Credit), before Tax		(16,000)	(17,000)	(17,000)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, for Net Gain (Loss), before Tax		(1,582,000)	(1,486,000)	(689,000)
Other Comprehensive Income (Loss), Finalization of Pension and Other Postretirement Benefit Plan Valuation, before Tax		0	(350,000)	0
Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Adjustment, before Tax		(1,724,000)	(3,137,000)	(9,356,000)
Components of Net Periodic Benefit Costs for Both Pension Plans
Defined Benefit Plan, Service Cost		8,200,000	7,949,000	6,335,000
Defined Benefit Plan, Interest Cost		2,908,000	2,496,000	2,191,000
Defined Benefit Plan, Expected Return on Plan Assets		(2,235,000)	(2,182,000)	(1,978,000)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)		16,000	17,000	17,000
Defined Benefit Plan, Amortization of Gains (Losses)		1,582,000	1,486,000	689,000
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements		0	350,000	0
Defined Benefit Plan, Net Periodic Benefit Cost		$ 10,471,000	$ 10,116,000	$ 7,254,000
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation and Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate		4.20%	4.70%	4.30%
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase		6.20%	6.20%	6.20%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate		4.70%	4.30%	5.00%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets	[1]	7.50%	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		$ 6,532,000
Defined Benefit Plan, Expected Future Benefit Payments in Year Two		3,256,000
Defined Benefit Plan, Expected Future Benefit Payments in Year Three		4,480,000
Defined Benefit Plan, Expected Future Benefit Payments in Year Four		4,778,000
Defined Benefit Plan, Expected Future Benefit Payments in Year Five		5,772,000
Defined Benefit Plan, Expected Future Benefit Payments in Five Fiscal Years Thereafter		38,708,000
Qualified Plan [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Defined Benefit Plan Plan Assets Returned		0
Non Qualified Plan [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Defined Benefit Plan, Fair Value of Plan Assets Beginning of Year		0	$ 0
Defined Benefit Plan, Fair Value of Plan Assets End of Year		0	0	$ 0
Defined Benefit Plan, Pension Plans with Accumulated Benefit Obligations in Excess of Plan Assets [Abstract]
Defined Benefit Plan, Fair Value of Plan Assets Beginning 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 Long-term Return on Assets			0.00%	0.00%

[1] 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.