Retirement and Postretirement Plans - Schedule of net benefit costs (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 0	$ 43	$ 0	$ 85
Interest cost	4,056	5,554	11,456	11,060
Expected return on plan assets	(5,472)	(6,762)	(15,453)	(13,433)
Amortization of net loss	142	0	400	0
Net periodic benefit (income) cost	(1,274)	(1,165)	(3,597)	(2,288)
Other Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	40	18	50	40
Interest cost	390	277	500	538
Expected return on plan assets	0	0	0	0
Amortization of net loss	0	0	0	0
Net periodic benefit (income) cost	$ 430	$ 295	$ 550	$ 578