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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Calculation of Basic Earnings Per Share:
Net Income	$ 17,801	$ 5,538	$ 6,387	$ 26,855	$ 26,101	$ 6,833	$ 6,046	$ 19,144	$ 56,580	$ 58,124	$ 44,675
Weighted shares outstanding - Basic (in shares)									16,369,616	16,336,789	15,570,539
Basic Earnings Per Share (in usd per share)	$ 1.09	$ 0.34	$ 0.39	$ 1.64	$ 1.60	$ 0.42	$ 0.37	$ 1.17	$ 3.46	$ 3.56	$ 2.87
Calculation of Diluted Earnings Per Share:
Net Income	$ 17,801	$ 5,538	$ 6,387	$ 26,855	$ 26,101	$ 6,833	$ 6,046	$ 19,144	$ 56,580	$ 58,124	$ 44,675
Reconciliation of Denominator:
Weighted shares outstanding - Basic (in shares)									16,369,616	16,336,789	15,570,539
Share-based Compensation									50,254	46,563	42,552
Adjusted denominator — Diluted									16,419,870	16,383,352	15,613,091
Diluted Earnings Per Share (in usd per share)	$ 1.08	$ 0.34	$ 0.39	$ 1.64	$ 1.59	$ 0.42	$ 0.37	$ 1.17	$ 3.45	$ 3.55	$ 2.86