Pensions and Other Post-retirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2,721	$ 2,634	$ 5,442	$ 5,268
Interest cost	4,572	4,702	9,144	9,404
Expected return on plan assets	(8,738)	(8,682)	(17,476)	(17,364)
Amortization of prior service cost	(4)	15	(8)	30
Recognized net actuarial losses	3,184	3,009	6,368	6,018
Settlements	34	20	68	40
Net periodic benefit cost, excluding below			3,538	3,396
Special termination charge			11,384	0
Net periodic benefit cost	1,769	1,698	14,922	3,396
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	106	106	212	212
Interest cost	237	237	474	474
Expected return on plan assets	0	0	0	0
Amortization of prior service cost	(105)	(105)	(210)	(210)
Recognized net actuarial losses	17	17	34	34
Settlements	0	0	0	0
Net periodic benefit cost, excluding below			510	510
Special termination charge			0	0
Net periodic benefit cost	$ 255	$ 255	$ 510	$ 510