Securities - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015 Investment		Mar. 31, 2014		Dec. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment Security Gains (Losses), net	$ 0.1	[1]	$ (4.0)	[1]
Net impairment losses recognized in earnings	0		3.9
Gross proceeds from sale of securities	102.0		199.7
Gross realized securities gains	0.1		0.3
Gross realized securities losses			0.4
Number of securities in an unrealized loss position	391
Total Fair Value	7,511.1				10,410.8
Total Unrealized Losses	49.2				87.6
U.S. Government Sponsored Agency
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Unrealized Losses	16.9
Others | Community Reinvestment Act CRA
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Unrealized Losses	19.0
Auction Rate Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Unrealized Losses	1.1
Corporate Debt
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Unrealized Losses	10.3
Percent of corporate debt portfolio	39.00%
Residential Mortgage-Backed
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Unrealized Losses	$ 0.5
Number of securities in an unrealized loss position for more than 12 months	1
Other Asset-Backed | Floating Rate Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Percentage of "other asset-backed" securities rated triple-A	100.00%
Credit Rating	AAA
Other Asset-Backed | Floating Rate Securities | Upper Limit
Schedule of Trading Securities and Other Trading Assets [Line Items]
Average life	5 years

[1] Changes in Other-Than-Temporary-Impairment (OTTI) Losses $ - $ (4.6 ) Noncredit-related OTTI Losses Recorded in/(Reclassified from) OCI - 0.7 Other Security Gains (Losses), net 0.1 (0.1 ) Investment Security Gains (Losses), net $ 0.1 $ (4.0 )