Securities - Additional Information (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) Securities	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) Securities
Investments, Debt and Equity Securities [Abstract]
Number of securities holdings | Securities	0		0
Shareholders' equity percentage	10.00%		10.00%
Securities	$ 274,883		$ 319,575
Tax provision related to net realized gains	$ 0	$ 484