Pension Plans and Other Postretirement Benefits - Contributions and Plan Assets (Details) - Pension Plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Contributions to employees' accounts	$ 0	$ 0	$ 0
Total assets at the end of the year	22,090	25,843	25,446	$ 27,667
Qualified plan | 401(k) plan
Defined Benefit Plan Disclosure [Line Items]
Contributions to employees' accounts	7,655	7,637	6,918
Total assets at the end of the year	167,767	180,514	177,352
Nonqualified plan
Defined Benefit Plan Disclosure [Line Items]
Contributions to employees' accounts	70	84	72
Total assets at the end of the year	$ 0	$ 0	$ 0