Pension and Other Postretirement Benefits - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pension Plans, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Service cost	$ 6,508	$ 6,309	$ 13,208	$ 12,684
Interest cost	6,909	7,006	13,909	14,131
Expected return on plan assets	(7,590)	(6,733)	(15,315)	(13,508)
Amortization of: Net actuarial loss	5,451	4,632	10,751	9,582
Amortization of: Prior service cost (gain)	110	112	210	212
Net periodic benefit cost	11,388	11,326	22,763	23,101
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Service cost	588	569	1,163	1,144
Interest cost	722	740	1,422	1,515
Expected return on plan assets	0	0	0	0
Amortization of: Net actuarial loss	169	154	394	354
Amortization of: Prior service cost (gain)	(540)	(540)	(1,090)	(1,090)
Net periodic benefit cost	$ 939	$ 923	$ 1,889	$ 1,923