Postretirement Benefit Plans - Summary of Post-Retirement Benefit Costs by Component (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 4,486	$ 3,894	$ 13,457	$ 11,682
Interest cost	5,552	5,430	16,653	16,294
Expected return on assets	(5,900)	(5,782)	(17,697)	(17,346)
Amortization:
Prior service cost	118	120	356	360
Actuarial (gain) loss	2,952	2,423	8,855	7,270
Direct recognition of expense	0	0	0	0
Net periodic benefit cost	7,208	6,085	21,624	18,260
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	0	0
Interest cost	249	212	749	636
Expected return on assets	0	0	0	0
Amortization:
Prior service cost	0	0	0	0
Actuarial (gain) loss	38	8	115	24
Direct recognition of expense	111	45	323	99
Net periodic benefit cost	$ 398	$ 265	$ 1,187	$ 759