Postretirement Benefit Plans - Summary of Post-Retirement Benefit Costs by Component (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 4,487	$ 3,894
Interest cost	5,550	5,432
Expected return on assets	(5,899)	(5,782)
Amortization:
Prior service cost	119	120
Actuarial (gain) loss	2,951	2,424
Direct recognition of expense	0	0
Net periodic benefit cost	7,208	6,088
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0
Interest cost	250	212
Expected return on assets	0	0
Amortization:
Prior service cost	0	0
Actuarial (gain) loss	39	8
Direct recognition of expense	96	34
Net periodic benefit cost	$ 385	$ 254