Postretirement Benefit Plans - Summary of Post-Retirement Benefit Costs by Component (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 4,484	$ 3,894	$ 8,971	$ 7,788
Interest cost	5,551	5,432	11,101	10,864
Expected return on assets	(5,898)	(5,782)	(11,797)	(11,564)
Amortization:
Prior service cost	119	120	238	240
Actuarial (gain) loss	2,952	2,423	5,903	4,847
Direct recognition of expense	0	0	0	0
Net periodic benefit cost	7,208	6,087	14,416	12,175
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	0	0
Interest cost	250	212	500	424
Expected return on assets	0	0	0	0
Amortization:
Prior service cost	0	0	0	0
Actuarial (gain) loss	38	8	77	16
Direct recognition of expense	116	20	212	54
Net periodic benefit cost	$ 404	$ 240	$ 789	$ 494