Retirement Plans - Net Periodic Pension Expense for Defined Benefit Pension Plans (Details) - Defined Benefit Pension Plans - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 18,129	$ 15,750	$ 17,999
Interest cost	9,899	19,617	21,820
Expected return on assets	(26,304)	(32,645)	(38,064)
Amortization of prior service credit	(903)	(886)	(935)
Recognized net actuarial loss	5,806	10,303	8,368
(Gain) loss on settlement	(406)	137,898	21,900
Net periodic pension expense	$ 6,221	$ 150,037	$ 31,088

Amount of gain (loss), recognized immediately as component of net periodic benefit (cost) credit, for change in value of benefit obligation or plan assets from experience different from that assumed, change in actuarial assumption, or consequence of temporarily deviating from substantive plan. Includes, but is not limited to, amount in excess of ten percent of greater of fair value of plan assets or benefit obligation.

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Amount of prior service cost (credit) recognized in net periodic benefit cost (credit) of defined benefit plan.

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Amount of expected return (loss) recognized in net periodic benefit (cost) credit, calculated based on expected long-term rate of return and market-related value of plan assets of defined benefit plan.

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Amount of cost recognized for passage of time related to defined benefit plan.

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Amount of net periodic benefit cost (credit) for defined benefit plan.

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Amount of gain (loss) recognized in net periodic benefit (cost) credit from irrevocable action relieving primary responsibility for benefit obligation and eliminating risk related to obligation and assets used to effect settlement.

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Amount of cost for actuarial present value of benefits attributed to service rendered by employee for defined benefit plan.

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