Securities Continuous Unrealized Loss Position (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	$ 3,458.0	$ 2,394.5
Less than 12 Months Unrealized Losses	9.2	6.2
12 Months or Longer Fair Value	452.5	1,431.3
12 Months or Longer Unrealized Losses	16.3	24.0
Total Fair Value	3,910.5	3,825.8
Total Unrealized Losses	25.5	30.2
U.S. Government Sponsored Agency
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	980.1	482.2
Less than 12 Months Unrealized Losses	2.2	1.0
12 Months or Longer Fair Value	278.2	1,171.8
12 Months or Longer Unrealized Losses	2.4	3.0
Total Fair Value	1,258.3	1,654.0
Total Unrealized Losses	4.6	4.0
Corporate Debt
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	760.2	441.5
Less than 12 Months Unrealized Losses	3.2	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	785.2	491.5
Total Unrealized Losses	3.3	2.1
Covered Bonds
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value		20.1
Less than 12 Months Unrealized Losses		0.1
Total Fair Value		20.1
Total Unrealized Losses		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
Residential Mortgage-Backed
Investments, Unrealized Loss Position [Line Items]
12 Months or Longer Fair Value	68.1	84.7
12 Months or Longer Unrealized Losses	7.9	10.8
Total Fair Value	68.1	84.7
Total Unrealized Losses	7.9	10.8
Other Asset-Backed
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	624.3	146.1
Less than 12 Months Unrealized Losses	0.3	0.1
12 Months or Longer Fair Value	25.5	40.0
12 Months or Longer Unrealized Losses	0.1	0.3
Total Fair Value	649.8	186.1
Total Unrealized Losses	0.4	0.4
Certificates of Deposit
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	1,081.9	1,178.8
Less than 12 Months Unrealized Losses	0.9	0.6
Total Fair Value	1,081.9	1,178.8
Total Unrealized Losses	0.9	0.6
Auction Rate Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	1.0	2.7
Less than 12 Months Unrealized Losses	0.1	0.3
12 Months or Longer Fair Value	15.5	41.0
12 Months or Longer Unrealized Losses	1.3	3.6
Total Fair Value	16.5	43.7
Total Unrealized Losses	1.4	3.9
Others
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	10.5	9.3
Less than 12 Months Unrealized Losses	2.5	1.9
12 Months or Longer Fair Value	40.2	43.8
12 Months or Longer Unrealized Losses	4.5	6.2
Total Fair Value	50.7	53.1
Total Unrealized Losses	$ 7.0	$ 8.1