Postretirement Benefit Plans - Summary of Post-Retirement Benefit Costs by Component (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 5,277	$ 4,487
Interest cost	5,493	5,550
Expected return on assets	(6,363)	(5,899)
Amortization:
Prior service cost	119	119
Actuarial (gain) loss	3,636	2,951
Direct recognition of expense	0	0
Net periodic benefit cost	8,162	7,208
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0
Interest cost	228	250
Expected return on assets	0	0
Amortization:
Prior service cost	0	0
Actuarial (gain) loss	23	39
Direct recognition of expense	140	96
Net periodic benefit cost	$ 391	$ 385