Stockholders' Equity (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Nov. 30, 2017	May 31, 2017	May 31, 2016	May 31, 2015
Stockholders' equity
Common stock, outstanding shares (in shares)	16,823,111		16,870,000
2018 Stock Purchase Plan
Stockholders' equity
Common stock purchased under the stock purchase plan (in shares)	66,724	0
Cost of shares of common stock acquired	$ 5,300,000	$ 0
2018 Stock Purchase Plan | Maximum
Stockholders' equity
Authorized amount of repurchase under the stock purchase plan (up to)				$ 25,000,000
2017 Stock Purchase Plan
Stockholders' equity
Common stock purchased under the stock purchase plan (in shares)	0	0
Cost of shares of common stock acquired	$ 0	$ 0
2017 Stock Purchase Plan | Maximum
Stockholders' equity
Authorized amount of repurchase under the stock purchase plan (up to)					$ 25,000,000
2016 Stock Purchase Plan
Stockholders' equity
Authorized amount of repurchase under the stock purchase plan (up to)						$ 25,000,000
Common stock purchased under the stock purchase plan (in shares)	0	85,354
Cost of shares of common stock acquired	$ 0	$ 6,800,000
2016 Stock Purchase Plan | Maximum
Stockholders' equity
Authorized amount of repurchase under the stock purchase plan (up to)							$ 25,000,000