Pension and Other Postretirement Benefits - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pension Plans, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Service cost	$ 7,100	$ 6,700
Interest cost	6,975	7,000
Expected return on plan assets	(8,075)	(7,725)
Amortization of: Net actuarial loss	6,725	5,300
Amortization of: Prior service cost (gain)	75	100
Net periodic benefit cost	12,800	11,375
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Service cost	575	575
Interest cost	650	700
Expected return on plan assets	0	0
Amortization of: Net actuarial loss	225	225
Amortization of: Prior service cost (gain)	(500)	(550)
Net periodic benefit cost	$ 950	$ 950