Pensions and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2,634	$ 2,904	$ 5,268	$ 5,808
Interest cost	4,702	4,593	9,404	9,186
Expected return on plan assets	(8,682)	(8,537)	(17,364)	(17,074)
Amortization of prior service cost	15	17	30	34
Recognized net actuarial losses	3,009	4,083	6,018	8,012
Settlements	20	33	40	66
Net periodic benefit cost	1,698	3,093	3,396	6,032
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	106	111	212	222
Interest cost	237	216	474	432
Expected return on plan assets	0	0	0	0
Amortization of prior service cost	(105)	(84)	(210)	(168)
Recognized net actuarial losses	17	7	34	14
Settlements	0	0	0	0
Net periodic benefit cost	$ 255	$ 250	$ 510	$ 500