Fair Value Measurements - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale securities portfolio, value	$ 27,476.5		$ 27,476.5		$ 28,643.5
Trading account securities	11.0		11.0		8.0
Other operating expenses	66.80	62.90	147.60	136.00
Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Number of available for sale securities portfolio	749		749		696
Available for sale securities portfolio, value	25,600.0		25,600.0		26,800.0
Loans | Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Financial Instrument, fair value	45.6	23.2	45.6	23.2
Change in specific allowance	1.9	14.2	0	4.7
Other Real Estate Owned | Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Financial Instrument, fair value	2.9	1.1	2.9	1.1
Other operating expenses	0.40	0.03	0.70	0.10
External Pricing | Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Number of available for sale securities portfolio	734		734		689
Available for sale securities portfolio, value	25,000.0		25,000.0		26,500.0
Trading account securities	11.0		11.0		8.0
Internal Pricing | Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Number of available for sale securities portfolio	15		15		7
Available for sale securities portfolio, value	$ 593.3		$ 593.3		$ 307.1