Securities - Additional Information (Detail) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) Securities	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) Securities	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) Securities
Investments, Debt and Equity Securities [Abstract]
Number of securities holdings | Securities	0		0		0
Shareholders' equity percentage	10.00%		10.00%		10.00%
Securities	$ 298,989		$ 298,989		$ 329,379
Tax provision related to net realized gains	$ 2	$ 0	$ 540	$ 352