Pensions and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2,634	$ 2,904
Interest cost	4,702	4,593
Expected return on plan assets	(8,682)	(8,537)
Amortization of prior service cost	15	17
Recognized net actuarial losses	3,009	3,929
Settlements	20	33
Net periodic benefit cost	1,698	2,939
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	106	111
Interest cost	237	216
Expected return on plan assets	0	0
Amortization of prior service cost	(105)	(84)
Recognized net actuarial losses	17	7
Settlements	0	0
Net periodic benefit cost	$ 255	$ 250