Accumulated Other Comprehensive (Loss) Income - Changes to Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 696,122	$ 1,071,140
Ending balance	342,968	868,938
Employee benefit costs
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(58,616)	(23,130)
Other comprehensive (loss) income before reclassifications	(14,154)	458
Amounts reclassified from accumulated other comprehensive (loss) income	4,156	532
Ending balance	$ (68,614)	$ (22,140)