Reconciliation of Amortized Cost to Fair Values of Securities Available for Sale (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Investment Holdings [Line Items]
Amortized Cost	$ 28,046.2	$ 28,382.3
Gross Unrealized Gains	125.0	174.2
Gross Unrealized Losses	63.5	163.7
Fair Value	28,107.7	28,392.8
U.S. Government
Investment Holdings [Line Items]
Amortized Cost	3,395.1	1,896.7
Gross Unrealized Gains	16.0	22.6
Gross Unrealized Losses	1.5	1.4
Fair Value	3,409.6	1,917.9
Obligations of States and Political Subdivisions
Investment Holdings [Line Items]
Amortized Cost	4.5	4.5
Gross Unrealized Gains	0.1	0.1
Fair Value	4.6	4.6
U.S. Government Sponsored Agency
Investment Holdings [Line Items]
Amortized Cost	16,114.2	17,495.2
Gross Unrealized Gains	85.4	80.7
Gross Unrealized Losses	25.2	47.9
Fair Value	16,174.4	17,528.0
Non-U.S. Government
Investment Holdings [Line Items]
Amortized Cost	309.3	307.0
Gross Unrealized Gains	1.2	3.6
Fair Value	310.5	310.6
Corporate Debt
Investment Holdings [Line Items]
Amortized Cost	3,393.5	3,615.2
Gross Unrealized Gains	4.7	10.5
Gross Unrealized Losses	33.4	101.2
Fair Value	3,364.8	3,524.5
Covered Bonds
Investment Holdings [Line Items]
Amortized Cost	1,966.9	1,898.9
Gross Unrealized Gains	12.2	50.9
Gross Unrealized Losses	0.2	5.9
Fair Value	1,978.9	1,943.9
Supranational and Non-U.S. Agency Bonds
Investment Holdings [Line Items]
Amortized Cost	360.0	410.0
Gross Unrealized Gains	1.4	1.7
Gross Unrealized Losses	1.2	1.7
Fair Value	360.2	410.0
Residential Mortgage-Backed
Investment Holdings [Line Items]
Amortized Cost	18.8	52.4
Gross Unrealized Gains	0.1	0.1
Gross Unrealized Losses	1.3	4.4
Fair Value	17.6	48.1
Other Asset-Backed
Investment Holdings [Line Items]
Amortized Cost	2,232.0	2,390.8
Gross Unrealized Gains	1.3	1.4
Gross Unrealized Losses	0.2	0.4
Fair Value	2,233.1	2,391.8
Auction Rate Securities
Investment Holdings [Line Items]
Amortized Cost	83.4	97.5
Gross Unrealized Gains	2.3	2.2
Gross Unrealized Losses	0.5	0.8
Fair Value	85.2	98.9
Others
Investment Holdings [Line Items]
Amortized Cost	168.5	214.1
Gross Unrealized Gains	0.3	0.4
Fair Value	$ 168.8	$ 214.5