Benefit Plans - Option Plans (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Additional disclosure
Intrinsic value	$ 7,700	$ 2,000	$ 60
Compensation expense	$ 927	$ 883	$ 514
Stock option
Number of Stock Options Outstanding
Options outstanding at the beginning of the year (in shares)	1,083,667
Granted (in shares)	210,977
Exercised (in shares)	(333,959)
Forfeited and expired (in shares)	(6,162)
Options outstanding at the end of the year (in shares)	954,523	1,083,667
Vested at the end of the year (in shares)	749,281
Weighted Average Exercise Price Per Share
Options outstanding at the beginning of the year (in dollars per share)	$ 12.61
Granted (in dollars per share)	20.40
Exercised (in dollars per share)	13.49
Forfeited and expired (in dollars per share)	34.68
Options outstanding at the end of the year (in dollars per share)	13.89	$ 12.61
Vested at the end of the year (in dollars per shares)	$ 12.47
Weighted average remaining contractual life
Outstanding at the end of the year	5 years 9 months 18 days
Vested at the end of the year	5 years 3 months 15 days
Additional disclosure
Aggregate intrinsic value, outstanding	$ 24,926
Aggregate Intrinsic value, vested and expected to vest	20,627
Unrecognized compensation expense	$ 814