Investment Securities - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013
Schedule of Available-for-Sale Securities [Line Items]
Weighted average life of securities	4 years 6 months
Securities, carrying value	$ 265.9	$ 399.0	$ 217.3
Maximum [Member]
Schedule of Available-for-Sale Securities [Line Items]
Weighted average life of securities	6 years