Postretirement Benefits Other Than Pensions - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 5,547	$ 6,717	$ 6,805
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service and interest cost	1,620	1,545	1,725
Prior service cost	92	92	92
Unrecognized net (gain) loss	(472)	(457)	(245)
Net periodic benefit cost	$ 1,240	$ 1,180	$ 1,572