Pension Benefits (Details) - USD ($)		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Defined Benefit Plan, service cost		$ 4,516,000	$ 5,582,000	$ 6,730,000
Defined Benefit Plan, interest cost		2,358,000	2,791,000	2,622,000
Defined Benefit Plan, actuarial loss		7,483,000	2,035,000
Defined Benefit Plan, benefits paid		(905,000)	(5,651,000)
Defined Benefit Plan, settlements		(10,702,000)	0
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Defined Benefit Plan, fair value of plan assets at beginning of year		35,634,000	30,100,000
Defined Benefit Plan, actual return on plan assets		2,837,000	3,985,000
Defined Benefit Plan, employer contribution		6,030,000	7,200,000	8,100,000
Defined Benefit Plan, benefits paid excluding settlements		(905,000)	(5,651,000)
Defined Benefit Plan, settlements		(10,702,000)	0
Defined Benefit Plan, fair value of plan assets at end of year		32,894,000	35,634,000	30,100,000
Defined Benefit Plan, funded status at end of year		(40,699,000)	(35,209,000)
Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets [Abstract]
Defined Benefit Plan, projected benefit obligation		73,593,000	70,843,000	66,086,000
Defined Benefit Plan, accumulated benefit obligation		63,934,000	60,877,000
Defined Benefit Plan, fair value of plan assets		32,894,000	35,634,000
Defined Benefit Plan, underfunded accumulated benefit obligation		$ 31,040,000	25,243,000
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		$ 17,328,000	14,406,000
Defined Benefit Plan, unrecognized prior service costs		14,000	31,000
Defined Benefit Plan, accumulated other comprehensive loss		17,342,000	14,437,000
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss) [Abstract]
Other Comprehensive Income (Loss), Defined Benefit Plan, net actuarial gain (loss)		(6,381,000)	377,000	4,329,000
Other Comprehensive Income (Loss), Defined Benefit Plan, amortization of prior service cost		17,000	17,000	18,000
Other Comprehensive Income (Loss), Defined Benefit Plan, amortization of net actuarial loss		950,000	958,000	1,327,000
Other Comprehensive Income (Loss), Defined Benefit Plan, settlements		2,509,000	0	0
Other Comprehensive Income (Loss), Defined Benefit Plan, total pension liability adjustment, pre-tax		(2,905,000)	1,352,000	5,674,000
Other Comprehensive Income (Loss), Defined Benefit Plan, tax (expense) benefit		626,000	(291,000)	(4,265,000)
Stockholders' Equity Attributable to Parent		2,016,160,000	2,748,994,000	2,920,322,000		$ 2,394,608,000
Other Comprehensive Income (Loss), Defined Benefit Plan, total pension liability adjustment, net	[1]	(2,279,000)	1,061,000	4,378,000
Components of Net Periodic Benefit Costs for Both Pension Plans
Defined Benefit Plan, service cost		4,516,000	5,582,000	6,730,000
Defined Benefit Plan, interest cost		2,358,000	2,791,000	2,622,000
Defined Benefit Plan, expected return on plan assets that reduces periodic pension benefit cost		(1,735,000)	(1,574,000)	(1,862,000)
Defined Benefit Plan, amortization of prior service cost		17,000	17,000	18,000
Defined Benefit Plan, amortization of net actuarial loss		950,000	958,000	1,327,000
Defined Benefit Plan, net periodic benefit cost		6,106,000	7,774,000	8,835,000
Defined Benefit Plan, settlements		2,509,000	0	0
Defined Benefit Plan, Total Net Benefit Cost		$ 8,615,000	$ 7,774,000	$ 8,835,000
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation and Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, discount rate		2.90%	3.60%
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, rate of compensation increase		4.40%	4.50%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, discount rate		3.60%	4.40%	3.80%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, expected return on plan assets		5.30%	5.00%	5.50%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, rate of compensation increase		4.50%	6.20%	6.20%
Defined Benefit Plan, Expected Future Employer Contributions [Abstract]
Defined Benefit Plan, Expected Future Contributions in Next Fiscal Year		$ 7,000,000.0
Future Benefit Payments
Defined Benefit Plan, Expected Future Benefit Payments in Year One		9,564,000
Defined Benefit Plan, Expected Future Benefit Payments in Year Two		3,769,000
Defined Benefit Plan, Expected Future Benefit Payments in Year Three		5,390,000
Defined Benefit Plan, Expected Future Benefit Payments in Year Four		4,765,000
Defined Benefit Plan, Expected Future Benefit Payments in Year Five		5,996,000
Defined Benefit Plan, Expected Future Benefit Payments in Five Fiscal Years Thereafter		24,132,000
Defined Benefit Plan, Plan Assets, Benefits Paid		$ (11,600,000)	$ (5,700,000)	$ (8,000,000.0)
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage		100.00%
Cumulative Effect, Period of Adoption, Adjustment
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss) [Abstract]
Stockholders' Equity Attributable to Parent					$ 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%	0.00%
Defined Benefit Plan, Equity Securities
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Defined Benefit Plan, fair value of plan assets at beginning of year		$ 13,134,000
Defined Benefit Plan, fair value of plan assets at end of year		$ 12,159,000	$ 13,134,000
Future Benefit Payments
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage		36.00%
Fixed Income Securities [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		$ 13,578,000
Defined Benefit Plan, fair value of plan assets at end of year		$ 8,202,000	13,578,000
Future Benefit Payments
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage		37.00%
Other Securities [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		$ 8,922,000
Defined Benefit Plan, fair value of plan assets at end of year		$ 12,533,000	8,922,000
Future Benefit Payments
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage		27.00%
Accounting Standards Update 2018-02 [Member] | Cumulative Effect, Period of Adoption, Adjustment
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss) [Abstract]
Stockholders' Equity Attributable to Parent		$ 0	$ 0	$ 2,969,000

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