Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Defined Benefit Plan Disclosure [Line Items]
Service cost, net	$ 4,065	$ 4,016	$ 8,032	$ 8,208
Interest cost on projected benefit obligations	3,682	3,208	7,387	6,508
Expected return on plan assets	(8,705)	(9,420)	(17,463)	(19,139)
Recognition of prior service cost	(1,622)	(1,820)	(3,339)	(3,707)
Recognition of actuarial losses/(gains)	5,482	6,507	11,088	13,205
Net periodic pension cost/(credit)	2,902	2,491	5,705	5,075
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost, net	266	272	532	544
Interest cost on projected benefit obligations	1,146	1,061	2,292	2,122
Expected return on plan assets	(1,472)	(1,732)	(2,944)	(3,464)
Recognition of prior service cost	0	0	0	0
Recognition of actuarial losses/(gains)	593	1,451	1,187	2,902
Net periodic pension cost/(credit)	533	1,052	1,067	2,104
Other Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost, net	3,799	3,744	7,500	7,664
Interest cost on projected benefit obligations	2,520	2,131	5,063	4,354
Expected return on plan assets	(7,218)	(7,688)	(14,519)	(15,675)
Recognition of prior service cost	(1,637)	(1,727)	(3,339)	(3,521)
Recognition of actuarial losses/(gains)	5,062	5,369	10,247	10,929
Net periodic pension cost/(credit)	2,526	1,829	4,952	3,751
Defined Benefit Plan, Expected Future Employer Contributions, Current Fiscal Year, Description	25,300		25,300
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost, net	0	0	0	0
Interest cost on projected benefit obligations	16	16	32	32
Expected return on plan assets	0	0	0	0
Recognition of prior service cost	0	(93)	0	(186)
Recognition of actuarial losses/(gains)	(173)	(313)	(346)	(626)
Net periodic pension cost/(credit)	$ (157)	$ (390)	$ (314)	$ (780)