Investment Securities - Schedule of Amortized Cost of Held-to-Maturity Investments (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity Securities, Original Cost	$ 119,499
Held-to-maturity Securities, Unrealized Gain at Transfer	3,559
Held-to-maturity Securities, Accretion	(171)
Held-to-maturity Securities, Amortized Cost	122,887	126,168
Transferred Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity Securities, Original Cost	118,204
Held-to-maturity Securities, Unrealized Gain at Transfer	3,559
Held-to-maturity Securities, Accretion	(171)
Held-to-maturity Securities, Amortized Cost	121,592
Other Held-to-Maturity Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity Securities, Original Cost	1,295
Held-to-maturity Securities, Amortized Cost	$ 1,295