Securities Continuous Unrealized Loss Position (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Investments, Unrealized Loss Position [Line Items]
Less Than 12 Months Fair Value	$ 712.4	$ 9,067.8
Less Than 12 Months Unrealized Losses	10.1	33.4
12 Months or Longer Fair Value	1,510.0	1,155.7
12 Months or Longer Unrealized Losses	21.7	51.6
Total Fair Value	2,222.4	10,223.5
Total Unrealized Losses	31.8	85.0
Government Sponsored Agency
Investments, Unrealized Loss Position [Line Items]
Less Than 12 Months Fair Value	165.8	5,492.5
Less Than 12 Months Unrealized Losses	0.4	14.1
12 Months or Longer Fair Value	1,251.3	470.1
12 Months or Longer Unrealized Losses	3.6	3.3
Total Fair Value	1,417.1	5,962.6
Total Unrealized Losses	4.0	17.4
Corporate Debt
Investments, Unrealized Loss Position [Line Items]
Less Than 12 Months Fair Value	129.9	1,027.5
Less Than 12 Months Unrealized Losses	0.3	4.1
12 Months or Longer Fair Value	49.9	123.6
12 Months or Longer Unrealized Losses	0.1	1.6
Total Fair Value	179.8	1,151.1
Total Unrealized Losses	0.4	5.7
Supranational Bonds
Investments, Unrealized Loss Position [Line Items]
Less Than 12 Months Fair Value	235.9	438.2
Less Than 12 Months Unrealized Losses	0.5	1.8
12 Months or Longer Fair Value		99.9
12 Months or Longer Unrealized Losses		0.1
Total Fair Value	235.9	538.1
Total Unrealized Losses	0.5	1.9
Residential Mortgage-Backed
Investments, Unrealized Loss Position [Line Items]
Less Than 12 Months Fair Value		4.7
Less Than 12 Months Unrealized Losses		0.9
12 Months or Longer Fair Value	95.5	158.8
12 Months or Longer Unrealized Losses	13.3	31.4
Total Fair Value	95.5	163.5
Total Unrealized Losses	13.3	32.3
Other Asset-Backed
Investments, Unrealized Loss Position [Line Items]
Less Than 12 Months Fair Value	112.1	824.6
Less Than 12 Months Unrealized Losses	0.1	2.3
12 Months or Longer Fair Value	68.8	205.7
12 Months or Longer Unrealized Losses	0.6	1.0
Total Fair Value	180.9	1,030.3
Total Unrealized Losses	0.7	3.3
Certificates of Deposit
Investments, Unrealized Loss Position [Line Items]
Less Than 12 Months Fair Value		1,019.9
Less Than 12 Months Unrealized Losses		0.3
Total Fair Value		1,019.9
Total Unrealized Losses		0.3
Auction Rate Securities
Investments, Unrealized Loss Position [Line Items]
Less Than 12 Months Fair Value	19.9	61.0
Less Than 12 Months Unrealized Losses	0.5	7.3
12 Months or Longer Fair Value	41.0	52.6
12 Months or Longer Unrealized Losses	3.8	5.2
Total Fair Value	60.9	113.6
Total Unrealized Losses	4.3	12.5
Others
Investments, Unrealized Loss Position [Line Items]
Less Than 12 Months Fair Value	48.8	146.3
Less Than 12 Months Unrealized Losses	8.3	2.1
12 Months or Longer Fair Value	3.5	45.0
12 Months or Longer Unrealized Losses	0.3	9.0
Total Fair Value	52.3	191.3
Total Unrealized Losses	8.6	11.1
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
Covered Bonds
Investments, Unrealized Loss Position [Line Items]
Less Than 12 Months Fair Value		50.4
Less Than 12 Months Unrealized Losses		0.4
Total Fair Value		50.4
Total Unrealized Losses		$ 0.4