Reconciliation of Amortized Cost to Fair Values of Securities Held to Maturity (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 4,211.7	$ 2,325.8
Gross Unrealized Gains	14.8	12.3
Gross Unrealized Losses	17.3	16.7
Fair Value	4,209.2	2,321.4
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	196.2	225.2
Gross Unrealized Gains	9.7	10.3
Gross Unrealized Losses
Fair Value	205.9	235.5
U.S. Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	32.1	35.9
Gross Unrealized Gains	1.1	1.1
Gross Unrealized Losses
Fair Value	33.2	37.0
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	176.1	197.3
Gross Unrealized Gains
Gross Unrealized Losses	0.1
Fair Value	176.0	197.3
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,531.8	698.1
Gross Unrealized Gains	0.1
Gross Unrealized Losses	0.1	0.2
Fair Value	1,531.8	697.9
Supranational, Sovereign and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,217.5	1,109.4
Gross Unrealized Gains	3.8	0.8
Gross Unrealized Losses	6.3	4.3
Fair Value	2,215.0	1,105.9
Others
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	58.0	59.9
Gross Unrealized Gains	0.1	0.1
Gross Unrealized Losses	10.8	12.2
Fair Value	$ 47.3	$ 47.8