Changes in Accumulated Other Comprehensive Loss by Component - Rollforward (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Changes to accumulated other comprehensive loss by component
Balance at the beginning of the period	$ (5,495)	$ (2,814)
Other comprehensive gain (loss) before reclassifications	(213)	(4,503)
Amounts reclassified from accumulated other comprehensive loss	719	209
Balance at the end of the period	(4,989)	(7,108)
Unrealized Net Loss on Interest Rate Swap
Changes to accumulated other comprehensive loss by component
Balance at the beginning of the period	(7,608)	(5,023)
Other comprehensive gain (loss) before reclassifications	(213)	(4,503)
Amounts reclassified from accumulated other comprehensive loss	777	266
Balance at the end of the period	(7,044)	(9,260)
Other Postretirement Benefit Liability
Changes to accumulated other comprehensive loss by component
Balance at the beginning of the period	2,113	2,209
Amounts reclassified from accumulated other comprehensive loss	(58)	(57)
Balance at the end of the period	$ 2,055	$ 2,152