Stock-Based Compensation, Restricted Shares (Details) - Restricted Shares - 2012 Directors Restricted Stock Plan - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Stock-Based Compensation
Shares available for the granting of additional stock options	109,150
Restricted share activity
Restricted shares outstanding at the beginning of the period (in shares)	14,873
Restricted shares outstanding at the end of the period (in shares)	14,873	14,873
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
Fair value at the grant date, restricted shares outstanding at the end of the period (in dollars per share)	$ 87.89	$ 87.89
Weighted Average Remaining Amortization Term (in years)	1 month 21 days	4 months 21 days
Total compensation cost related to non-vested restricted shares not yet recognized (in dollars)	$ 0.2
Weighted average expected amortization period	1 month 21 days