Stock-Based Compensation, Restricted Shares (Details) - Restricted Shares - 2012 Directors Restricted Stock Plan - USD ($) $ / shares in Units, $ in Millions	3 Months Ended			9 Months Ended	12 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2021	Dec. 31, 2020
Stock-Based Compensation
Remaining shares available for the granting of stock options under plan	141,359			141,359
Restricted share activity
Restricted shares outstanding at the beginning of the period (in shares)	19,224	26,350	26,350	26,350
Shares vested and issued (in shares)		(27,272)
Restricted shares granted (in shares)	1,837	20,146	0
Restricted shares outstanding at the end of the period (in shares)	21,061	19,224	26,350	21,061	26,350
Fair Value at Grant Date
Fair value at the grant date, restricted shares outstanding at the beginning of the period (in dollars per share)	$ 66.59	$ 45.55	$ 45.55	$ 45.55
Fair value of restricted shares vested and issued (in dollars per share)		45.80
Fair value of restricted shares granted (in dollars per share)	70.39	65.96	0
Fair value at the grant date, restricted shares outstanding at the end of the period (in dollars per share)	$ 66.92	$ 66.59	$ 45.55	$ 66.92	$ 45.55
Weighted Average Remaining Amortization Term (in years)	7 months 13 days	10 months 17 days	1 month 17 days		4 months 17 days
Total compensation cost related to non-vested restricted shares not yet recognized (in dollars)	$ 0.9			$ 0.9
Weighted average expected amortization period				7 months 13 days