Share-Based Compensation - Deferred Compensation Plan (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
SERP expense	$ 7,872	$ 6,865	$ 5,900
SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
SERP expense	$ 512	$ 539	$ 547
Treasury shares issued to fund SERP expense (in shares)	18,000	22,000	14,000
SERP trust account balance at December 31	$ 54,729	$ 43,952	$ 39,766
Unrealized gain (loss) recorded in SERP liability account	$ 9,200	$ 7,353	$ (1,469)