Summary of Significant Accounting Policies - Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Numerator
Numerator for basic and diluted earnings per share-net income									$ 14,313	$ 31,903	$ 51,943
Denominator
Basic weighted average common shares outstanding (in shares)									13,754,516	14,768,625	15,102,890
Dilutive effect of stock options (in shares)									384,217	510,026	805,321
Diluted weighted average common shares outstanding (in shares)									14,138,733	15,278,651	15,908,211
Basic net income per share
Basic (in dollars per share)	$ 0.08	$ (0.17)	$ 0.49	$ 0.64	$ 0.86	$ 0.94	$ 0.18	$ 0.18	$ 1.04	$ 2.16	$ 3.44
Diluted net income per share
Diluted (in dollars per share)	$ 0.08	$ (0.17)	$ 0.48	$ 0.62	$ 0.84	$ 0.91	$ 0.17	$ 0.17	$ 1.01	$ 2.09	$ 3.27