Pension and Other Postretirement Benefits - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Pension Plans, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Service cost	$ 6,700	$ 6,375
Interest cost	7,000	7,125
Expected return on plan assets	(7,725)	(6,775)
Amortization of: Net actuarial loss	5,300	4,950
Amortization of: Prior service cost (gain)	100	100
Net periodic benefit cost	11,375	11,775
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Service cost	575	575
Interest cost	700	775
Expected return on plan assets	0	0
Amortization of: Net actuarial loss	225	200
Amortization of: Prior service cost (gain)	(550)	(550)
Net periodic benefit cost	$ 950	$ 1,000