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, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Stock-Based Compensation
Shares available for the granting of additional stock options	81,671			81,671
Restricted share activity
Restricted shares outstanding at the beginning of the period (in shares)	12,418	12,516	12,516	12,516
Shares vested and issued (in shares)		(12,516)
Restricted shares granted (in shares)	757	12,418	0
Restricted shares outstanding at the end of the period (in shares)	13,175	12,418	12,516	13,175	12,516
Fair Value at Grant Date
Fair value at the grant date, restricted shares outstanding at the beginning of the period (in dollars per share)	$ 90.21	$ 89.49	$ 89.49	$ 89.49
Fair value of restricted shares vested and issued (in dollars per share)		89.49
Fair value of restricted shares granted (in dollars per share)	105.83	90.21	0
Fair value at the grant date, restricted shares outstanding at the end of the period (in dollars per share)	$ 91.11	$ 90.21	$ 89.49	$ 91.11	$ 89.49
Weighted Average Remaining Amortization Term (in years)	7 months 17 days	10 months 17 days	1 month 17 days		4 months 20 days
Total compensation cost related to non-vested restricted shares not yet recognized (in dollars)	$ 0.8			$ 0.8
Weighted average expected amortization period				7 months 17 days