Pensions and Other Post-retirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2,891	$ 2,721	$ 8,673	$ 8,163
Interest cost	4,218	4,572	12,656	13,716
Expected return on plan assets	(9,097)	(8,738)	(27,289)	(26,214)
Amortization of prior service cost	(4)	(4)	(16)	(12)
Recognized net actuarial losses	3,454	3,184	10,360	9,552
Settlement/curtailment loss (credit)	26	34	80	102
Net periodic benefit cost, excluding below			4,464	5,307
Special termination charge			0	11,384
Net periodic benefit cost (a)	1,488	1,769	4,464	16,691
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	92	96	276	308
Interest cost	199	204	595	678
Expected return on plan assets	0	0	0	0
Amortization of prior service cost	(101)	(6)	(303)	(216)
Recognized net actuarial losses	188	15	564	49
Settlement/curtailment loss (credit)	0	0	0	0
Net periodic benefit cost, excluding below			1,132	819
Special termination charge			0	0
Net periodic benefit cost (a)	$ 378	$ 309	$ 1,132	$ 819