Reconciliation of Amortized Cost to Fair Values of Securities Held to Maturity (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 5,248.3	$ 4,170.8
Gross Unrealized Gains	19.5	26.4
Gross Unrealized Losses	40.3	21.1
Fair Value	5,227.5	4,176.1
U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	26.0
Gross Unrealized Gains	0.0
Gross Unrealized Losses	0.0
Fair Value	26.0
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	89.2	121.9
Gross Unrealized Gains	5.2	7.4
Gross Unrealized Losses	0.0	0.0
Fair Value	94.4	129.3
U.S. Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	9.9	18.4
Gross Unrealized Gains	0.7	1.1
Gross Unrealized Losses	0.0	0.0
Fair Value	10.6	19.5
Covered Bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	892.4
Gross Unrealized Gains	0.4
Gross Unrealized Losses	1.9
Fair Value	890.9
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,118.0	1,281.6
Gross Unrealized Gains	4.8	6.6
Gross Unrealized Losses	0.5	0.4
Fair Value	1,122.3	1,287.8
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	691.6	924.3
Gross Unrealized Gains	0.1	0.1
Gross Unrealized Losses	0.1	0.1
Fair Value	691.6	924.3
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,326.2
Gross Unrealized Gains	8.3
Gross Unrealized Losses	0.9
Fair Value	2,333.6
Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost		1,745.8
Gross Unrealized Gains		10.9
Gross Unrealized Losses		0.5
Fair Value		1,756.2
Others
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	95.0	78.8
Gross Unrealized Gains	0.0	0.3
Gross Unrealized Losses	36.9	20.1
Fair Value	$ 58.1	$ 59.0