Postretirement Benefit Cost - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Retirement Benefits [Abstract]
Service cost	$ 2,005	$ 1,714
Interest cost	544	521
Expected return on plan assets	(524)	(334)
Net periodic benefit cost	$ 2,025	$ 1,901