Reconciliation of Amortized Cost to Fair Values of Securities Held to Maturity (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 2,931.7	$ 2,382.0
Gross Unrealized Gains	19.5	21.5
Gross Unrealized Losses	7.9	8.7
Fair Value	2,943.3	2,394.8
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	292.7	329.3
Gross Unrealized Gains	15.6	17.2
Fair Value	308.3	346.5
U.S. Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	80.2	112.9
Gross Unrealized Gains	2.7	3.8
Fair Value	82.9	116.7
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	210.8	205.0
Fair Value	210.8	205.0
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,069.9	1,667.6
Gross Unrealized Gains	0.2	0.2
Gross Unrealized Losses	0.9	0.6
Fair Value	2,069.2	1,667.2
Others
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	278.1	67.2
Gross Unrealized Gains	1.0	0.3
Gross Unrealized Losses	7.0	8.1
Fair Value	$ 272.1	$ 59.4