Postretirement Benefits - Summary of Post-Retirement Benefit Costs by Component (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 5,277	$ 4,484	$ 10,554	$ 8,971
Interest cost	5,493	5,551	10,986	11,101
Expected return on assets	(6,363)	(5,898)	(12,726)	(11,797)
Amortization:
Prior service cost	119	119	238	238
Actuarial (gain) loss	3,636	2,952	7,272	5,903
Direct recognition of expense	0	0	0	0
Net periodic benefit cost	8,162	7,208	16,324	14,416
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	0	0
Interest cost	228	250	456	500
Expected return on assets	0	0	0	0
Amortization:
Prior service cost	0	0	0	0
Actuarial (gain) loss	23	38	46	77
Direct recognition of expense	85	116	225	212
Net periodic benefit cost	$ 336	$ 404	$ 727	$ 789