Securities Continuous Unrealized Loss Position (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	$ 9,989.0	$ 2,394.5
Less than 12 Months Unrealized Losses	115.6	6.2
12 Months or Longer Fair Value	475.3	1,431.3
12 Months or Longer Unrealized Losses	15.8	24.0
Total Fair Value	10,464.3	3,825.8
Total Unrealized Losses	131.4	30.2
U.S. Government Sponsored Agency
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	4,111.5	482.2
Less than 12 Months Unrealized Losses	38.4	1.0
12 Months or Longer Fair Value	320.2	1,171.8
12 Months or Longer Unrealized Losses	2.5	3.0
Total Fair Value	4,431.7	1,654.0
Total Unrealized Losses	40.9	4.0
Corporate Debt
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	2,132.7	441.5
Less than 12 Months Unrealized Losses	61.2	2.0
12 Months or Longer Fair Value	25.0	50.0
12 Months or Longer Unrealized Losses	0.1	0.1
Total Fair Value	2,157.7	491.5
Total Unrealized Losses	61.3	2.1
Covered Bonds
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	205.2	20.1
Less than 12 Months Unrealized Losses	4.5	0.1
Total Fair Value	205.2	20.1
Total Unrealized Losses	4.5	0.1
Supranational Bonds
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value		113.8
Less than 12 Months Unrealized Losses		0.2
Total Fair Value		113.8
Total Unrealized Losses		0.2
Supranational, Sovereign and Non-U.S. Agency Bonds
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	57.7
Less than 12 Months Unrealized Losses	2.0
Total Fair Value	57.7
Total Unrealized Losses	2.0
Residential Mortgage-Backed
Investments, Unrealized Loss Position [Line Items]
12 Months or Longer Fair Value	58.5	84.7
12 Months or Longer Unrealized Losses	5.5	10.8
Total Fair Value	58.5	84.7
Total Unrealized Losses	5.5	10.8
Other Asset-Backed
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	1,901.2	146.1
Less than 12 Months Unrealized Losses	3.9	0.1
12 Months or Longer Fair Value	16.3	40.0
12 Months or Longer Unrealized Losses	0.1	0.3
Total Fair Value	1,917.5	186.1
Total Unrealized Losses	4.0	0.4
Certificates of Deposit
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	1,056.9	1,178.8
Less than 12 Months Unrealized Losses	0.4	0.6
Total Fair Value	1,056.9	1,178.8
Total Unrealized Losses	0.4	0.6
Auction Rate Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	22.7	2.7
Less than 12 Months Unrealized Losses	0.1	0.3
12 Months or Longer Fair Value	16.0	41.0
12 Months or Longer Unrealized Losses	1.1	3.6
Total Fair Value	38.7	43.7
Total Unrealized Losses	1.2	3.9
Others
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	501.1	9.3
Less than 12 Months Unrealized Losses	5.1	1.9
12 Months or Longer Fair Value	39.3	43.8
12 Months or Longer Unrealized Losses	6.5	6.2
Total Fair Value	540.4	53.1
Total Unrealized Losses	$ 11.6	$ 8.1