Retirement Plans and Other Postretirement Benefits - Contributions and Plan Assets (Details) - Pension Plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Contributions to employees' accounts	$ 0	$ 0	$ 0
Total assets at the end of the year	23,164	22,090	25,843	$ 25,446
Qualified plan | 401(k) plan
Defined Benefit Plan Disclosure [Line Items]
Contributions to employees' accounts	8,233	7,655	7,637
Total assets at the end of the year	206,247	167,767	180,514
Nonqualified plan
Defined Benefit Plan Disclosure [Line Items]
Contributions to employees' accounts	58	70	84
Total assets at the end of the year	$ 0	$ 0	$ 0