Investment Securities - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015 positions	Mar. 31, 2014	Dec. 31, 2014 positions
Investment Identifier [Line Items]
Amount of impairment losses reclassified out of accumulated other comprehensive income into earnings	$ 0	$ 0
Total number of securities in an unrealized loss position	69		109
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	9,184,000		13,891,000
Securities with carrying amounts were pledged	$ 739,500,000		$ 755,500,000