Pension Plans and Other Postretirement Benefits - Contributions and Plan Assets (Details) - Pension Plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Contributions to employees' accounts	$ 0	$ 0	$ 0
Total assets at the end of the year	25,843	25,446	27,667	$ 31,408
Qualified plan | 401(k) plan
Defined Benefit Plan Disclosure [Line Items]
Contributions to employees' accounts	7,637	6,918	6,466
Total assets at the end of the year	180,514	177,352	161,956
Qualified plan | Qualified defined contribution plan
Defined Benefit Plan Disclosure [Line Items]
Contributions to employees' accounts	0	0	0
Total assets at the end of the year	0	0	9,118
Nonqualified plan
Defined Benefit Plan Disclosure [Line Items]
Contributions to employees' accounts	84	72	122
Total assets at the end of the year	$ 0	$ 0	$ 0