Reconciliation of Amortized Cost to Fair Values of Securities Held to Maturity (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 2,766.1	$ 2,382.0
Gross Unrealized Gains	13.6	21.5
Gross Unrealized Losses	11.9	8.7
Fair Value	2,767.8	2,394.8
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	263.0	329.3
Gross Unrealized Gains	11.9	17.2
Gross Unrealized Losses
Fair Value	274.9	346.5
U.S. Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	57.8	112.9
Gross Unrealized Gains	1.4	3.8
Gross Unrealized Losses
Fair Value	59.2	116.7
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	197.7	205.0
Gross Unrealized Gains
Gross Unrealized Losses
Fair Value	197.7	205.0
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,658.4	1,667.6
Gross Unrealized Gains		0.2
Gross Unrealized Losses	0.4	0.6
Fair Value	1,658.0	1,667.2
Others
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	589.2	67.2
Gross Unrealized Gains	0.3	0.3
Gross Unrealized Losses	11.5	8.1
Fair Value	$ 578.0	$ 59.4