Pension and Other Postretirement Benefits - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Pension Plans, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Service cost	$ 6,375	$ 6,500
Interest cost	7,125	6,350
Expected return on plan assets	(6,775)	(7,025)
Amortization of: Net actuarial loss	4,950	4,924
Amortization of: Prior service cost (gain)	100	113
Net periodic benefit cost	11,775	10,862
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Service cost	575	675
Interest cost	775	750
Expected return on plan assets	0	0
Amortization of: Net actuarial loss	200	251
Amortization of: Prior service cost (gain)	(550)	(545)
Net periodic benefit cost	$ 1,000	$ 1,131