Pension Plan and Postretirement Benefits Other Than Pension - Plan assets fair value (Details) - Pension Benefits - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	$ 162,999	$ 170,881	$ 144,529	$ 173,885
Total investment securities
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	160,275	169,752
Cash equivalents
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	465	66,779
Domestic
Pension Plan and Postretirement Benefits Other than Pension
Total fair value		49,540
International
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	4	31,431
U.S. treasuries
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	46,415	6,455
Corporate bonds
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	91,521	587
Foreign Bonds
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	21,870	6,591
Gold bullion
Pension Plan and Postretirement Benefits Other than Pension
Total fair value		8,369
Cash
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	2,724	1,129
Level 1 | Total investment securities
Pension Plan and Postretirement Benefits Other than Pension
Total fair value		62,798
Level 1 | Domestic
Pension Plan and Postretirement Benefits Other than Pension
Total fair value		49,540
Level 1 | International
Pension Plan and Postretirement Benefits Other than Pension
Total fair value		4,889
Level 1 | Gold bullion
Pension Plan and Postretirement Benefits Other than Pension
Total fair value		8,369
Level 2 | Total investment securities
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	160,275	106,954
Level 2 | Cash equivalents
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	465	66,779
Level 2 | International
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	4	26,542
Level 2 | U.S. treasuries
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	46,415	6,455
Level 2 | Corporate bonds
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	91,521	587
Level 2 | Foreign Bonds
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	$ 21,870	$ 6,591