Accumulated Other Comprehensive (Loss) Income - Changes to Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 696,122	$ 1,071,140
Ending balance	653,835	1,080,644
Employee benefit costs
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(58,616)	(23,130)
Other comprehensive income (loss) before reclassifications	(6,004)	0
Amounts reclassified from accumulated other comprehensive (loss) income	1,994	177
Ending balance	$ (62,626)	$ (22,953)