Investment Securities - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Schedule of Available-for-Sale Securities [Line Items]
Securities, carrying value	$ 426.6	$ 286.6	$ 295.7
Debt Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Number of securities, Total	346
Number of securities in unrealized loss position	48
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Schedule of Available-for-Sale Securities [Line Items]
Number of securities in unrealized loss position	35
State, County and Municipal Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Number of securities in unrealized loss position	11
U.S. Government Sponsored Agencies [Member]
Schedule of Available-for-Sale Securities [Line Items]
Number of securities in unrealized loss position	1
Corporate Debt Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Number of securities in unrealized loss position	1