Postretirement Benefits - Projected and Accumulated Benefit Obligation (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Plans with assets less than projected benefit obligations
Projected benefit obligation	$ 886,165	$ 951,666
Plan assets	767,569	743,900
Plans with assets less than accumulated benefit obligations
Accumulated benefit obligation		767,527
Plan assets		$ 743,900
Supplemental employee retirement plan (SERP) | Nonqualified Plan | Unfunded plan
Plans with assets less than accumulated benefit obligations
Accumulated benefit obligation	18,908
Plan assets	$ 0