Pension and Postretirement Benefits - Amounts recognized in accumulated other comprehensive income (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Amounts recognized in accumulated other comprehensive income
Total	$ 11,265	$ 13,476
Pension Plans | Funded Plans
Amounts recognized in accumulated other comprehensive income
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Gains (Losses), before Tax	9,202	9,442
Prior service credit	0	0
Subtotal	9,202	9,442
Less tax effect	(3,490)	(3,581)
Total	5,712	5,861
Pension Plans | Unfunded Pension Plans
Amounts recognized in accumulated other comprehensive income
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Gains (Losses), before Tax	2,806	4,020
Prior service credit	0	0
Subtotal	2,806	4,020
Less tax effect	(1,064)	(1,525)
Total	1,742	2,495
US Postretirement Benefit Plans
Amounts recognized in accumulated other comprehensive income
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Gains (Losses), before Tax	3,795	5,571
Prior service credit	(2,173)	(2,729)
Subtotal	1,622	2,842
Less tax effect	(615)	(1,078)
Total	1,007	1,764
US Postretirement and Pension Plan
Amounts recognized in accumulated other comprehensive income
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Gains (Losses), before Tax	15,803	19,033
Prior service credit	(2,173)	(2,729)
Subtotal	13,630	16,304
Less tax effect	(5,169)	(6,184)
Total	$ 8,461	$ 10,120