Postretirement Benefits - Summary of Post-Retirement Benefit Costs by Component (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Retirement Benefits [Abstract]
Service cost	$ 4,982	$ 5,277
Interest cost	5,964	5,493
Expected return on assets	(6,966)	(6,363)
Amortization:
Prior service cost	158	119
Actuarial (gain) loss	1,894	3,636
Net periodic benefit cost	$ 6,032	$ 8,162