Pension Plan and Postretirement Benefits Other Than Pension - Plan assets fair value (Details) - Pension Benefits - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	$ 183,903	$ 162,999	$ 170,881	$ 144,529
Total investment securities
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	182,526	160,275
Cash equivalents
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	91,989	465
International
Pension Plan and Postretirement Benefits Other than Pension
Total fair value		4
U.S. treasuries
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	19,311	46,415
Corporate bonds
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	62,313	91,521
Foreign Bonds
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	8,913	21,870
Cash
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	1,377	2,724
Level 2 | Total investment securities
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	182,526	160,275
Level 2 | Cash equivalents
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	91,989	465
Level 2 | International
Pension Plan and Postretirement Benefits Other than Pension
Total fair value		4
Level 2 | U.S. treasuries
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	19,311	46,415
Level 2 | Corporate bonds
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	62,313	91,521
Level 2 | Foreign Bonds
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	$ 8,913	$ 21,870