Reconciliation of Amortized Cost to Fair Values of Securities Held to Maturity (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 8,921.1	$ 5,248.3
Gross Unrealized Gains	29.2	19.5
Gross Unrealized Losses	45.2	40.3
Fair Value	8,905.1	5,227.5
U.S. Government Obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	15.0	26.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	15.0	26.0
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	63.6	89.2
Gross Unrealized Gains	2.7	5.2
Gross Unrealized Losses	0.0	0.0
Fair Value	66.3	94.4
Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	7.4	9.9
Gross Unrealized Gains	0.5	0.7
Gross Unrealized Losses	0.0	0.0
Fair Value	7.9	10.6
Corporate Debt
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	231.2
Gross Unrealized Gains	0.2
Gross Unrealized Losses	0.4
Fair Value	231.0
Covered Bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,051.6	892.4
Gross Unrealized Gains	10.1	0.4
Gross Unrealized Losses	3.7	1.9
Fair Value	2,058.0	890.9
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	3,517.5	1,118.0
Gross Unrealized Gains	4.9	4.8
Gross Unrealized Losses	2.3	0.5
Fair Value	3,520.1	1,122.3
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	606.0	691.6
Gross Unrealized Gains	0.2	0.1
Gross Unrealized Losses	0.1	0.1
Fair Value	606.1	691.6
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,154.7	2,326.2
Gross Unrealized Gains	10.5	8.3
Gross Unrealized Losses	2.8	0.9
Fair Value	2,162.4	2,333.6
Other Asset-Backed
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	143.4
Gross Unrealized Gains	0.1
Gross Unrealized Losses	0.0
Fair Value	143.5
Other
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	130.7	95.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	35.9	36.9
Fair Value	$ 94.8	$ 58.1