Equity, Restricted Shares (Details) - Restricted Shares - 2012 Directors Restricted Stock Plan - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stock-Based Compensation
Restricted shares available for grant	94,846
Restricted share activity
Restricted shares outstanding at the beginning of the period (in shares)	14,873	12,905	16,905
Restricted shares granted (in shares)	14,304	15,014	12,905
Shares vested and issued (in shares)	(16,661)	(13,046)	(16,905)
Restricted shares outstanding at the end of the period (in shares)	12,516	14,873	12,905	16,905
Fair Value at Grant Date
Fair value at the grant date, restricted shares outstanding at the beginning of the period (in dollars per share)	$ 87.89	$ 73.25	$ 57.99
Fair value of restricted shares vested and issued (in dollars per share)	88.07	73.35	57.99
Fair value of restricted shares granted (in dollars per share)	89.49	87.84	73.25
Fair value at the grant date, restricted shares outstanding at the end of the period (in dollars per share)	$ 89.49	$ 87.89	$ 73.25	$ 57.99
Weighted Average Remaining Amortization Term (in years)	4 months 20 days	4 months 20 days	4 months 13 days	4 months 17 days
Total fair value of restricted share awards vested (in dollars)	$ 1.5	$ 1.0	$ 1.0
Total compensation cost related to non-vested restricted shares not yet recognized (in dollars)	$ 0.4
Weighted average expected amortization period	4 months 20 days