Share-Based Payments - Stock Option Plan 2007 (Details) - Stock Option Plan 2007 - USD ($)	3 Months Ended
	Mar. 31, 2017	Sep. 12, 2007
Share-Based Payments
Number of awards that can be granted		29,889
Vested option rights outstanding	$ 1,087,000
Weighted average exercise price in $ per share
Outstanding balance at beginning of year (in dollars per share)	$ 7.32
Exercised (in dollars per share)	7.32
Outstanding balance at end of year (in dollars per share)	7.32
Vested and exercisable balance at end of year (in dollars per share)	$ 7.32
Options
Outstanding at beginning of year (in shares)	10,996
Exercised (in shares)	(5,156)
Outstanding at end of year (in shares)	5,840
Vested and exercisable at end of year (in shares)	5,712
Aggregate Intrinsic value
Outstanding at end of year (in dollars)	$ 1,112,000
Vested and exercisable at end of year (in dollars)	1,086,000
Total intrinsic value of options exercised	$ 943,000
Other disclosures
Weighted-average remaining contractual life	2 months 12 days
Share-based compensation expense (in dollars)	$ 15,000
Weighted average of share price at the date of exercise of options (in dollars per share)	$ 97.67
Options
Share-Based Payments
Vesting period	4 years
Minimum percentage of shares or asset sale to permit exercise	50.00%
Options | First year
Share-Based Payments
Vesting period	1 year
Percentage that vests during the period	25.00%
Options | Second year
Share-Based Payments
Vesting period	2 years
Percentage that vests during the period	25.00%
Options | Third and fourth years
Share-Based Payments
Vesting period	4 years
Percentage that vests during the period	50.00%
Monthly vesting percentage	2.083%