Investment Securities - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013 Security	Dec. 31, 2012 Security	Dec. 31, 2011
Investments Debt And Equity Securities [Abstract]
Transfer of securities with fair value	$ 95.4
Accumulated other comprehensive income included net pre-tax unrealized losses	5.9
Number of securities in loss position	88	39
Securities with estimated market values	204.3	121.4
Proceeds from sales of securities	45.8	125.4	156.8
Gross gains realized	0.5	4.3	1.1
Gross losses realized	$ 0.2	$ 0	$ 0.6