Regulatory Matters - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)
Banking and Thrift [Abstract]
Percentage of surplus	Not less than 50%
Amount of dividends available for payment after restrictions	$ 507.9
Tier 1 capital conservation buffer of risk-weighted assets	2.50%
Capital conservation buffer of risk-weighted assets, 2016	0.625%
Tier 1 capital conservation buffer of risk-weighted assets, 2017	0.625%
Tier 1 capital conservation buffer of risk-weighted assets, 2018	0.625%
Tier 1 capital conservation buffer of risk-weighted assets, 2019	0.625%