Pension and Other Postretirement Benefits - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Pension Plans, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Service cost	$ 6,402	$ 5,551	$ 19,206	$ 16,654
Interest cost	6,301	6,704	18,902	20,113
Expected return on plan assets	(7,075)	(6,656)	(21,223)	(19,968)
Amortization of: Net actuarial loss	5,007	4,410	15,021	13,229
Amortization of: Prior service cost (gain)	113	238	339	714
Settlement loss			0	789
Net periodic benefit cost	10,748	10,247	32,245	31,531
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Service cost	626	614	1,878	1,841
Interest cost	731	834	2,194	2,502
Expected return on plan assets	0	0	0	0
Amortization of: Net actuarial loss	261	317	783	951
Amortization of: Prior service cost (gain)	(545)	(545)	(1,635)	(1,635)
Settlement loss			0	0
Net periodic benefit cost	$ 1,073	$ 1,220	$ 3,220	$ 3,659