Retirement Plans and Other Postretirement Benefits - Contributions and Plan Assets (Details) - Pension Plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Contributions to employees' accounts	$ 0	$ 0	$ 0
Total assets at the end of the year	21,945	23,164	22,090	$ 25,843
Qualified Plan | 401(k) plan
Defined Benefit Plan Disclosure [Line Items]
Contributions to employees' accounts	8,187	8,233	7,655
Total assets at the end of the year	228,449	206,247	167,767
Nonqualified Plan
Defined Benefit Plan Disclosure [Line Items]
Contributions to employees' accounts	68	58	70
Total assets at the end of the year	$ 0	$ 0	$ 0