Pension and Other Postretirement Benefits - Amounts Recognized in Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Amounts recognized in the balance sheets:
Accrued benefit liability, noncurrent	$ (160,400)	$ (136,227)
Pension Benefits
Pension And Other Employee Benefit Plans [Line Items]
Funded status, end of year	(131,123)	(105,626)
Amounts recognized in the balance sheets:
Prepaid benefit cost	4,780	5,542
Accrued benefit liability, current	(3,558)	(3,000)
Accrued benefit liability, noncurrent	(132,345)	(108,168)
Net funded status	(131,123)	(105,626)
Other Benefits
Pension And Other Employee Benefit Plans [Line Items]
Funded status, end of year	(29,498)	(29,470)
Amounts recognized in the balance sheets:
Prepaid benefit cost	0	0
Accrued benefit liability, current	(1,443)	(1,411)
Accrued benefit liability, noncurrent	(28,055)	(28,059)
Net funded status	$ (29,498)	$ (29,470)