Earnings Per Share - Calculations of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Calculation of Basic Earnings Per Share:
Net Income	$ 26,101	$ 6,833	$ 6,046	$ 19,144	$ 11,863	$ 4,416	$ 8,029	$ 20,367	$ 58,124	$ 44,675	$ 41,140
Weighted shares outstanding - Basic (in shares)									16,336,789	15,570,539	15,094,423
Basic Earnings Per Share (in usd per share)	$ 1.60	$ 0.42	$ 0.37	$ 1.17	$ 0.73	$ 0.29	$ 0.52	$ 1.33	$ 3.56	$ 2.87	$ 2.73
Calculation of Diluted Earnings Per Share:
Net Income	$ 26,101	$ 6,833	$ 6,046	$ 19,144	$ 11,863	$ 4,416	$ 8,029	$ 20,367	$ 58,124	$ 44,675	$ 41,140
Reconciliation of Denominator:
Weighted shares outstanding - Basic (in shares)									16,336,789	15,570,539	15,094,423
Share-based Compensation									46,563	42,552	48,950
Adjusted denominator — Diluted									16,383,352	15,613,091	15,143,373
Diluted Earnings Per Share (in usd per share)	$ 1.59	$ 0.42	$ 0.37	$ 1.17	$ 0.73	$ 0.29	$ 0.52	$ 1.33	$ 3.55	$ 2.86	$ 2.72