Investment Securities - Additional Information (Detail) (USD $)	9 Months Ended	12 Months Ended
	Sep. 30, 2014 Investmentsecurities	Dec. 31, 2013 Investmentsecurities
Investment Securities [Line Items]
Investment securities sold	0	0
Investment securities carrying value	$ 81,828,000	$ 62,064,000
Mortgage-backed securities	$ 506,776,000
Life of mortgage-backed securities	5 years 8 months 12 days
Obligations of U.S. government corporations and agencies [Member]
Investment Securities [Line Items]
Debt securities	13
Percentage of aggregate depreciation in unrealized losses	1.79%
Obligations of states and political subdivisions [Member]
Investment Securities [Line Items]
Debt securities	10
Percentage of aggregate depreciation in unrealized losses	1.62%
Corporate debt securities [Member]
Investment Securities [Line Items]
Debt securities	0