Securities - Reconciliation of Amortized Cost to Fair Value of Securities Available for Sale (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Investment Holdings [Line Items]
Total	$ 33,864.8	$ 35,631.9
GROSS UNREALIZED GAINS	66.6	78.7
GROSS UNREALIZED LOSSES	189.3	130.8
FAIR VALUE	33,742.1	35,579.8
U.S. Government Obligations
Investment Holdings [Line Items]
Total	5,714.4	7,514.5
GROSS UNREALIZED GAINS	18.0	22.4
GROSS UNREALIZED LOSSES	32.1	14.3
FAIR VALUE	5,700.3	7,522.6
Obligations of States and Political Subdivisions
Investment Holdings [Line Items]
Total	749.9	890.8
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	3.5	5.6
FAIR VALUE	746.4	885.2
Government Sponsored Agency
Investment Holdings [Line Items]
Total	18,745.3	17,914.1
GROSS UNREALIZED GAINS	39.9	49.3
GROSS UNREALIZED LOSSES	108.6	70.6
FAIR VALUE	18,676.6	17,892.8
Non-U.S. Government
Investment Holdings [Line Items]
Total	179.1	420.0
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	1.9	2.1
FAIR VALUE	177.2	417.9
Corporate Debt
Investment Holdings [Line Items]
Total	3,013.7	3,787.4
GROSS UNREALIZED GAINS	2.2	2.6
GROSS UNREALIZED LOSSES	22.9	24.8
FAIR VALUE	2,993.0	3,765.2
Covered Bonds
Investment Holdings [Line Items]
Total	879.0	1,148.6
GROSS UNREALIZED GAINS	1.0	0.8
GROSS UNREALIZED LOSSES	4.4	5.5
FAIR VALUE	875.6	1,143.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Investment Holdings [Line Items]
Total	1,819.8	1,343.6
GROSS UNREALIZED GAINS	4.0	0.9
GROSS UNREALIZED LOSSES	3.8	3.8
FAIR VALUE	1,820.0	1,340.7
Other Asset-Backed
Investment Holdings [Line Items]
Total	2,297.7	2,083.7
GROSS UNREALIZED GAINS	1.5	2.7
GROSS UNREALIZED LOSSES	7.9	1.3
FAIR VALUE	2,291.3	2,085.1
Auction Rate
Investment Holdings [Line Items]
Total	4.4	5.0
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	0.1	0.3
FAIR VALUE	4.3	4.7
Commercial Mortgage-Backed
Investment Holdings [Line Items]
Total	439.2	474.1
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	4.1	2.5
FAIR VALUE	435.1	471.6
Other
Investment Holdings [Line Items]
Total	22.3	50.1
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	0.0	0.0
FAIR VALUE	$ 22.3	$ 50.1