Pensions and Other Post-retirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2,721	$ 2,634	$ 8,163	$ 7,902
Interest cost	4,572	4,702	13,716	14,106
Expected return on plan assets	(8,738)	(8,682)	(26,214)	(26,046)
Amortization of prior service cost	(4)	17	(12)	47
Recognized net actuarial losses	3,184	3,010	9,552	9,028
Settlements	34	17	102	57
Net periodic benefit cost, excluding below			5,307	5,094
Special termination charge			11,384	0
Net periodic benefit cost	1,769	1,698	16,691	5,094
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	96	106	308	318
Interest cost	204	237	678	711
Expected return on plan assets	0	0	0	0
Amortization of prior service cost	(6)	(105)	(216)	(315)
Recognized net actuarial losses	15	17	49	51
Settlements	0	0	0	0
Net periodic benefit cost, excluding below			819	765
Special termination charge			0	0
Net periodic benefit cost	$ 309	$ 255	$ 819	$ 765