Postretirement Benefit Plans - Summary of Post-Retirement Benefit Costs by Component (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3,894	$ 3,990	$ 11,682	$ 11,971
Interest cost	5,430	5,003	16,294	15,009
Expected return on assets	(5,782)	(5,323)	(17,346)	(15,969)
Amortization:
Prior service cost	120	81	360	244
Actuarial (gain) loss	2,423	3,533	7,270	10,601
Direct recognition of expense	0	0	0	0
Net periodic benefit cost	6,085	7,284	18,260	21,856
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	0	0
Interest cost	212	203	636	610
Expected return on assets	0	0	0	0
Amortization:
Prior service cost	0	0	0	0
Actuarial (gain) loss	8	30	24	90
Direct recognition of expense	45	166	99	493
Net periodic benefit cost	$ 265	$ 399	$ 759	$ 1,193