Pension and Other Postretirement Benefits - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Pension Plans, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Service cost	$ 6,342	$ 6,402	$ 19,026	$ 19,206
Interest cost	7,066	6,301	21,197	18,902
Expected return on plan assets	(6,754)	(7,075)	(20,262)	(21,223)
Amortization of: Net actuarial loss	4,791	5,007	14,373	15,021
Amortization of: Prior service cost (gain)	106	113	318	339
Net periodic benefit cost	11,551	10,748	34,652	32,245
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Service cost	572	626	1,716	1,878
Interest cost	757	731	2,272	2,194
Expected return on plan assets	0	0	0	0
Amortization of: Net actuarial loss	177	261	531	783
Amortization of: Prior service cost (gain)	(545)	(545)	(1,635)	(1,635)
Net periodic benefit cost	$ 961	$ 1,073	$ 2,884	$ 3,220