Fair Value Disclosures - Assets and Liabilities Measured at Fair Value (Details) - Recurring - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
SERP
Assets:
SERP	$ 2,420	$ 2,433
Interest rate swaps
Liabilities:
Interest rate derivatives	(1,689)	(10,255)
Level 1 | SERP
Assets:
SERP	0	0
Level 1 | Interest rate swaps
Liabilities:
Interest rate derivatives	0	0
Level 2 | SERP
Assets:
SERP	2,420	2,433
Level 2 | Interest rate swaps
Liabilities:
Interest rate derivatives	(1,689)	(10,255)
Level 3 | SERP
Assets:
SERP	0	0
Level 3 | Interest rate swaps
Liabilities:
Interest rate derivatives	$ 0	$ 0