Securities Continuous Unrealized Loss Position (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Investments, Unrealized Loss Position [Line Items]
Less Than 12 Months Fair Value	$ 9,223.5	$ 9,067.8
Less Than 12 Months Unrealized Losses	19.2	33.4
12 Months or Longer Fair Value	1,322.3	1,155.7
12 Months or Longer Unrealized Losses	47.6	51.6
Total Fair Value	10,545.8	10,223.5
Total Unrealized Losses	66.8	85.0
Obligations of States and Political Subdivisions
Investments, Unrealized Loss Position [Line Items]
Less Than 12 Months Fair Value		2.7
Less Than 12 Months Unrealized Losses		0.1
Total Fair Value		2.7
Total Unrealized Losses		0.1
Government Sponsored Agency
Investments, Unrealized Loss Position [Line Items]
Less Than 12 Months Fair Value	5,111.9	5,492.5
Less Than 12 Months Unrealized Losses	9.4	14.1
12 Months or Longer Fair Value	528.9	470.1
12 Months or Longer Unrealized Losses	3.5	3.3
Total Fair Value	5,640.8	5,962.6
Total Unrealized Losses	12.9	17.4
Corporate Debt
Investments, Unrealized Loss Position [Line Items]
Less Than 12 Months Fair Value	835.6	1,027.5
Less Than 12 Months Unrealized Losses	1.1	4.1
12 Months or Longer Fair Value	99.8	123.6
12 Months or Longer Unrealized Losses	0.2	1.6
Total Fair Value	935.4	1,151.1
Total Unrealized Losses	1.3	5.7
Covered Bonds
Investments, Unrealized Loss Position [Line Items]
Less Than 12 Months Fair Value	283.5	50.4
Less Than 12 Months Unrealized Losses	2.0	0.4
Total Fair Value	283.5	50.4
Total Unrealized Losses	2.0	0.4
Supranational Bonds
Investments, Unrealized Loss Position [Line Items]
Less Than 12 Months Fair Value	262.0	438.2
Less Than 12 Months Unrealized Losses	1.4	1.8
12 Months or Longer Fair Value	149.8	99.9
12 Months or Longer Unrealized Losses	0.2	0.1
Total Fair Value	411.8	538.1
Total Unrealized Losses	1.6	1.9
Residential Mortgage-Backed
Investments, Unrealized Loss Position [Line Items]
Less Than 12 Months Fair Value	1.0	4.7
Less Than 12 Months Unrealized Losses	0.9	0.9
12 Months or Longer Fair Value	131.3	158.8
12 Months or Longer Unrealized Losses	23.0	31.4
Total Fair Value	132.3	163.5
Total Unrealized Losses	23.9	32.3
Other Asset-Backed
Investments, Unrealized Loss Position [Line Items]
Less Than 12 Months Fair Value	497.1	824.6
Less Than 12 Months Unrealized Losses	1.6	2.3
12 Months or Longer Fair Value	260.1	205.7
12 Months or Longer Unrealized Losses	1.1	1.0
Total Fair Value	757.2	1,030.3
Total Unrealized Losses	2.7	3.3
Certificates of Deposit
Investments, Unrealized Loss Position [Line Items]
Less Than 12 Months Fair Value	1,210.8	1,019.9
Less Than 12 Months Unrealized Losses	0.7	0.3
Total Fair Value	1,210.8	1,019.9
Total Unrealized Losses	0.7	0.3
Auction Rate Securities
Investments, Unrealized Loss Position [Line Items]
Less Than 12 Months Fair Value	6.3	61.0
Less Than 12 Months Unrealized Losses	0.5	7.3
12 Months or Longer Fair Value	106.9	52.6
12 Months or Longer Unrealized Losses	11.8	5.2
Total Fair Value	113.2	113.6
Total Unrealized Losses	12.3	12.5
Others
Investments, Unrealized Loss Position [Line Items]
Less Than 12 Months Fair Value	265.9	146.3
Less Than 12 Months Unrealized Losses	1.5	2.1
12 Months or Longer Fair Value	45.5	45.0
12 Months or Longer Unrealized Losses	7.8	9.0
Total Fair Value	311.4	191.3
Total Unrealized Losses	9.3	11.1
U.S. Government
Investments, Unrealized Loss Position [Line Items]
Less Than 12 Months Fair Value	749.4
Less Than 12 Months Unrealized Losses	0.1
Total Fair Value	749.4
Total Unrealized Losses	$ 0.1