Postretirement Benefit Plans - Summary of Post-Retirement Benefit Costs by Component (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3,894	$ 3,991	$ 7,788	$ 7,981
Interest cost	5,432	5,004	10,864	10,006
Expected return on assets	(5,782)	(5,323)	(11,564)	(10,646)
Amortization:
Prior service cost	120	82	240	163
Actuarial (gain) loss	2,423	3,534	4,847	7,068
Direct recognition of expense	0	0	0	0
Net periodic benefit cost	6,087	7,288	12,175	14,572
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	0	0
Interest cost	212	204	424	407
Expected return on assets	0	0	0	0
Amortization:
Prior service cost	0	0	0	0
Actuarial (gain) loss	8	37	16	60
Direct recognition of expense	20	151	54	327
Net periodic benefit cost	$ 240	$ 392	$ 494	$ 794