Postretirement Benefits - Summary of Post-Retirement Benefit Costs by Component (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 5,277	$ 4,486	$ 15,831	$ 13,457
Interest cost	5,496	5,552	16,482	16,653
Expected return on assets	(6,363)	(5,900)	(19,089)	(17,697)
Amortization:
Prior service cost	119	118	357	356
Actuarial (gain) loss	3,636	2,952	10,908	8,855
Direct recognition of expense	0	0	0	0
Net periodic benefit cost	8,165	7,208	24,489	21,624
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	0	0
Interest cost	228	249	684	749
Expected return on assets	0	0	0	0
Amortization:
Prior service cost	0	0	0	0
Actuarial (gain) loss	23	38	69	115
Direct recognition of expense	80	111	305	323
Net periodic benefit cost	$ 331	$ 398	$ 1,058	$ 1,187