Pensions and Other Post-retirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2,721	$ 2,634
Interest cost	4,572	4,702
Expected return on plan assets	(8,738)	(8,682)
Amortization of prior service cost	(4)	15
Recognized net actuarial losses	3,184	3,009
Settlements	34	20
Net periodic benefit cost, excluding below	1,769	1,698
Special termination charge	11,384	0
Net periodic benefit cost	13,153	1,698
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	106	106
Interest cost	237	237
Expected return on plan assets	0	0
Amortization of prior service cost	(105)	(105)
Recognized net actuarial losses	17	17
Settlements	0	0
Net periodic benefit cost, excluding below	255	255
Special termination charge	0	0
Net periodic benefit cost	$ 255	$ 255