Pension Plan and Postretirement Benefits Other Than Pension - Plan assets fair value (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension Plan and Postretirement Benefits Other than Pension
Amount of transfer between levels	$ 0	$ 0
Level 3
Pension Plan and Postretirement Benefits Other than Pension
Total fair value			$ 3,402
Pension Benefits
Pension Plan and Postretirement Benefits Other than Pension
Amount of transfer between levels	0	0
Total fair value	144,529	173,885	$ 175,548	$ 170,430
Pension Benefits | Equity securities | Domestic
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	71,159	92,037
Pension Benefits | Equity securities | International
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	24,622	25,822
Pension Benefits | Equity derivatives
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	12	280
Pension Benefits | Mortgage-backed securities
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	10	11
Pension Benefits | U.S. treasuries
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	4,801
Pension Benefits | Corporate bonds
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	526	8,113
Pension Benefits | Foreign Bonds
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	8,897
Pension Benefits | Gold bullion
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	8,420	5,226
Pension Benefits | Total investment securities
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	118,447	131,489
Pension Benefits | Cash and other
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	26,082	42,396
Pension Benefits | Level 1 | Equity securities | Domestic
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	71,159	92,037
Pension Benefits | Level 1 | Equity securities | International
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	24,622	25,822
Pension Benefits | Level 1 | Gold bullion
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	8,420	5,226
Pension Benefits | Level 1 | Total investment securities
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	104,201	123,085
Pension Benefits | Level 2 | Equity derivatives
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	12	280
Pension Benefits | Level 2 | Mortgage-backed securities
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	10	11
Pension Benefits | Level 2 | U.S. treasuries
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	4,801
Pension Benefits | Level 2 | Corporate bonds
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	526	8,113
Pension Benefits | Level 2 | Foreign Bonds
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	8,897
Pension Benefits | Level 2 | Total investment securities
Pension Plan and Postretirement Benefits Other than Pension
Total fair value	$ 14,246	$ 8,404