Share-based Compensation - Stock Options and Restricted Stock Activity (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Stock options
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Dividend yield (as a percent)	0.00%			2.85%
Expected life of option (in years)	0 years			3 years 5 months 24 days
Risk-free interest rate (as a percent)	0.00%			2.48%
Expected stock price volatility (as a percent)	0.00%			22.00%
Options Outstanding
Balance at beginning of period (in shares)	1,137,017	1,548,719	1,548,719
Granted (in shares)	0		6,000
Exercised (in shares)	0		(316,302)
Lapsed or cancelled (in shares)	(5,999)		(101,400)
Balance at end of period (in shares)	1,131,018		1,137,017	1,548,719
Options exercisable at end of period (in shares)	994,045		783,035
Total fair value of options granted during the period	$ 0		$ 84,068
Weighted Average Exercise Price
Balance at beginning of year (in dollars per share)	$ 103.54	$ 101.48	$ 101.48
Granted (in dollars per share)	0.00		97.91
Exercised (in dollars per share)	0.00		90.22
Lapsed or cancelled (in dollars per share)	110.56		113.22
Balance at end of period (in dollars per share)	103.50		103.54	$ 101.48
Options exercisable at end of period (in dollars per share)	$ 103.00		$ 101.28
Restricted Stock Awards
Options Outstanding
Total fair value of options granted during the period	$ 474,480	$ 13,440,503
Summary of restricted stock
Balance at beginning of year (in shares)	3,452,016	3,298,216	3,298,216
Granted (in shares)	6,000	162,900
Cancelled (in shares)	(5,950)	(9,100)
Balance at end of period (in shares)	3,452,066		3,452,016	3,298,216
Vested during the period (in shares)	110,274	92,114