Reconciliation of Amortized Cost to Fair Values of Securities Held to Maturity (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 4,196.7	$ 2,325.8
Gross Unrealized Gains	17.4	12.3
Gross Unrealized Losses	19.0	16.7
Fair Value	4,195.1	2,321.4
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	138.8	225.2
Gross Unrealized Gains	8.5	10.3
Fair Value	147.3	235.5
U.S. Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	22.8	35.9
Gross Unrealized Gains	1.1	1.1
Fair Value	23.9	37.0
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,094.1	722.0
Gross Unrealized Gains	3.7	0.8
Gross Unrealized Losses	0.7	1.1
Fair Value	1,097.1	721.7
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,479.5	698.1
Gross Unrealized Gains	0.2
Gross Unrealized Losses	0.1	0.2
Fair Value	1,479.6	697.9
Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,393.9	584.7
Gross Unrealized Gains	3.9
Gross Unrealized Losses	1.9	3.2
Fair Value	1,395.9	581.5
Others
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	67.6	59.9
Gross Unrealized Gains		0.1
Gross Unrealized Losses	16.3	12.2
Fair Value	$ 51.3	$ 47.8