Earnings Per Share - Schedule of Earnings Per Share, Basic and Diluted (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Earnings Per Share [Abstract]
Net income	$ 20,299	$ 21,036	$ 62,626	$ 64,822
Weighted average shares outstanding - basic and diluted	5,115,982	5,114,701	5,115,339	5,113,877
Net income per common share - basic and diluted	$ 3.97	$ 4.11	$ 12.24	$ 12.68