Pension and Other Postretirement Benefits - Fair Values of Pension Plan Assets by Asset Category (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	$ 361,802	$ 355,726
U.S. equities fund
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	86,059	131,563
Non-U.S. equities fund
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	61,630	54,496
Government bond fund
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	98,418	74,219
Corporate bond fund
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	82,434	40,940
Fixed income fund
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	7,320	35,895
Other investments
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	17,367	9,462
Cash & equivalents
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	8,574	9,151
Quoted Prices in Active Markets for Identical Assets (Level 1)
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	11,844	8,909
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. equities fund
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	3,012	2,793
Quoted Prices in Active Markets for Identical Assets (Level 1) | Non-U.S. equities fund
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	5,602	5,949
Quoted Prices in Active Markets for Identical Assets (Level 1) | Government bond fund
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate bond fund
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed income fund
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other investments
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash & equivalents
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	3,230	167
Significant Observable Inputs (Level 2)
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	12,922	44,300
Significant Observable Inputs (Level 2) | U.S. equities fund
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	0	0
Significant Observable Inputs (Level 2) | Non-U.S. equities fund
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	0	0
Significant Observable Inputs (Level 2) | Government bond fund
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	772	745
Significant Observable Inputs (Level 2) | Corporate bond fund
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	12,150	9,854
Significant Observable Inputs (Level 2) | Fixed income fund
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	0	33,701
Significant Observable Inputs (Level 2) | Other investments
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	0	0
Significant Observable Inputs (Level 2) | Cash & equivalents
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	0	0
Fair Value Measured at Net Asset Value Per Share
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	337,036	302,517
Fair Value Measured at Net Asset Value Per Share | U.S. equities fund
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	83,047	128,770
Fair Value Measured at Net Asset Value Per Share | Non-U.S. equities fund
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	56,028	48,547
Fair Value Measured at Net Asset Value Per Share | Government bond fund
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	97,646	73,474
Fair Value Measured at Net Asset Value Per Share | Corporate bond fund
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	70,284	31,086
Fair Value Measured at Net Asset Value Per Share | Fixed income fund
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	7,320	2,194
Fair Value Measured at Net Asset Value Per Share | Other investments
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	17,367	9,462
Fair Value Measured at Net Asset Value Per Share | Cash & equivalents
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	$ 5,344	$ 8,984