Reconciliation of Amortized Cost to Fair Value of Securities Available for Sale (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Investment Holdings [Line Items]
Total	$ 35,631.9	$ 32,367.8
Gross Unrealized Gains	78.7	50.3
Gross Unrealized Losses	130.8	100.2
Fair Value	35,579.8	32,317.9
U.S. Government Obligations
Investment Holdings [Line Items]
Total	7,514.5	6,180.4
Gross Unrealized Gains	22.4	3.4
Gross Unrealized Losses	14.3	5.5
Fair Value	7,522.6	6,178.3
Obligations of States and Political Subdivisions
Investment Holdings [Line Items]
Total	890.8	36.4
Gross Unrealized Gains	0.0	0.1
Gross Unrealized Losses	5.6	0.1
Fair Value	885.2	36.4
Government Sponsored Agency
Investment Holdings [Line Items]
Total	17,914.1	16,370.5
Gross Unrealized Gains	49.3	42.8
Gross Unrealized Losses	70.6	46.5
Fair Value	17,892.8	16,366.8
Non-U.S. Government
Investment Holdings [Line Items]
Total	420.0	309.5
Gross Unrealized Gains	0.0	0.1
Gross Unrealized Losses	2.1	0.1
Fair Value	417.9	309.5
Corporate Debt
Investment Holdings [Line Items]
Total	3,787.4	3,744.4
Gross Unrealized Gains	2.6	0.9
Gross Unrealized Losses	24.8	33.1
Fair Value	3,765.2	3,712.2
Covered Bonds
Investment Holdings [Line Items]
Total	1,148.6	1,873.3
Gross Unrealized Gains	0.8	1.8
Gross Unrealized Losses	5.5	4.9
Fair Value	1,143.9	1,870.2
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Investment Holdings [Line Items]
Total	1,343.6	860.9
Gross Unrealized Gains	0.9	0.5
Gross Unrealized Losses	3.8	2.0
Fair Value	1,340.7	859.4
Other Asset-Backed
Investment Holdings [Line Items]
Total	2,083.7	2,504.2
Gross Unrealized Gains	2.7	0.1
Gross Unrealized Losses	1.3	4.2
Fair Value	2,085.1	2,500.1
Auction Rate
Investment Holdings [Line Items]
Total	5.0	16.7
Gross Unrealized Gains	0.0	0.5
Gross Unrealized Losses	0.3	0.1
Fair Value	4.7	17.1
Commercial Mortgage-Backed
Investment Holdings [Line Items]
Total	474.1	378.1
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	2.5	3.7
Fair Value	471.6	374.4
Other
Investment Holdings [Line Items]
Total	50.1	93.4
Gross Unrealized Gains	0.0	0.1
Gross Unrealized Losses	0.0	0.0
Fair Value	$ 50.1	$ 93.5