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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.

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- Definition Weighted average rate for present value of future retirement benefits cash flows, used to determine benefit obligation of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(1) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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- Definition Weighted average rate of return on plan assets, reflecting average rate of earnings expected on existing plan assets and expected contributions, used to determine net periodic benefit cost of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(3) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e2919-114920 + Details
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- Definition Weighted average rate of compensation increase used to determine net periodic benefit cost of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(2) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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- Definition Amount of payment to participant of defined benefit plan which decreases benefit obligation. For pension plan, payment includes, but is not limited to, pension benefits and death benefits. For other postretirement plan, payment includes, but is not limited to, prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(6) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 60 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6414203&loc=d3e39716-114964 + Details
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- Definition Amount of payment, which decreases benefit obligation of defined benefit plan, for irrevocable action relieving primary responsibility for benefit obligation and eliminating risk for obligation and assets used to effect settlement. Transaction constituting settlement includes, but is not limited to, making lump-sum cash payment to participant in exchange for their rights to receive specified benefits and purchasing nonparticipating annuity contract. Excludes decreases to benefit obligation for remeasurement due to settlement. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(10) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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