Pensions and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2,634	$ 2,904	$ 7,902	$ 8,712
Interest cost	4,702	4,593	14,106	13,779
Expected return on plan assets	(8,682)	(8,537)	(26,046)	(25,611)
Amortization of prior service cost	17	17	47	51
Recognized net actuarial losses	3,010	3,929	9,028	11,787
Settlements	17	33	57	99
Net periodic benefit cost	1,698	2,939	5,094	8,817
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	106	111	318	333
Interest cost	237	216	711	648
Expected return on plan assets	0	0	0	0
Amortization of prior service cost	(105)	(84)	(315)	(252)
Recognized net actuarial losses	17	7	51	21
Settlements	0	0	0	0
Net periodic benefit cost	$ 255	$ 250	$ 765	$ 750