Pension and Other Postretirement Benefits - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Pension Plans, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Service cost	$ 6,309	$ 6,304	$ 12,684	$ 12,804
Interest cost	7,006	6,251	14,131	12,601
Expected return on plan assets	(6,733)	(7,123)	(13,508)	(14,148)
Amortization of: Net actuarial loss	4,632	5,090	9,582	10,014
Amortization of: Prior service cost (gain)	112	113	212	226
Net periodic benefit cost	11,326	10,635	23,101	21,497
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Service cost	569	577	1,144	1,252
Interest cost	740	713	1,515	1,463
Expected return on plan assets	0	0	0	0
Amortization of: Net actuarial loss	154	271	354	522
Amortization of: Prior service cost (gain)	(540)	(545)	(1,090)	(1,090)
Net periodic benefit cost	$ 923	$ 1,016	$ 1,923	$ 2,147