Retirement Plans - Summary of Categories in Fair Value Hierarchy (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Fair values of pension plan assets
Defined Benefit Plan, Plan Assets, Amount	$ 287,270	$ 282,696
Not Leveled
Fair values of pension plan assets
Defined Benefit Plan, Plan Assets, Amount	227,092	244,871
Quoted Prices in Active Markets (Level 1)
Fair values of pension plan assets
Defined Benefit Plan, Plan Assets, Amount	53,386	30,286
Significant Other Observable Inputs (Level 2)
Fair values of pension plan assets
Defined Benefit Plan, Plan Assets, Amount	0	0
Significant Unobservable Inputs (Level 3)
Fair values of pension plan assets
Defined Benefit Plan, Plan Assets, Amount	6,792	7,539
Equity Securities
Fair values of pension plan assets
Defined Benefit Plan, Plan Assets, Amount	7	24,586
Equity Securities | Quoted Prices in Active Markets (Level 1)
Fair values of pension plan assets
Defined Benefit Plan, Plan Assets, Amount	7	24,586
Bond Funds - Government
Fair values of pension plan assets
Defined Benefit Plan, Plan Assets, Amount	53,239	33,991
Bond Funds - Government | Not Leveled
Fair values of pension plan assets
Defined Benefit Plan, Plan Assets, Amount	53,239	33,991
Bond Funds - Other
Fair values of pension plan assets
Defined Benefit Plan, Plan Assets, Amount	173,853	207,901
Bond Funds - Other | Not Leveled
Fair values of pension plan assets
Defined Benefit Plan, Plan Assets, Amount	173,853	207,901
Real Estate
Fair values of pension plan assets
Defined Benefit Plan, Plan Assets, Amount	0	2,979
Real Estate | Not Leveled
Fair values of pension plan assets
Defined Benefit Plan, Plan Assets, Amount		2,979
Cash and Cash Equivalents
Fair values of pension plan assets
Defined Benefit Plan, Plan Assets, Amount	53,379	5,700
Cash and Cash Equivalents | Quoted Prices in Active Markets (Level 1)
Fair values of pension plan assets
Defined Benefit Plan, Plan Assets, Amount	53,379	5,700
Partnership
Fair values of pension plan assets
Defined Benefit Plan, Plan Assets, Amount	6,792	7,539
Partnership | Significant Unobservable Inputs (Level 3)
Fair values of pension plan assets
Defined Benefit Plan, Plan Assets, Amount	$ 6,792	$ 7,539	$ 9,172