Earnings Per Share - Schedule of Earnings Per Share, Basic and Diluted (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Earnings Per Share [Abstract]
Net income	$ 20,660	$ 21,767	$ 42,327	$ 43,786
Weighted average shares outstanding - basic and diluted	5,115,320	5,113,844	5,115,012	5,113,461
Net income per common share - basic and diluted	$ 4.04	$ 4.26	$ 8.28	$ 8.56