Securities Continuous Unrealized Loss Position (Detail) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	$ 8,487.7	$ 7,101.7
Less than 12 Months Unrealized Losses	25.8	26.5
12 Months or Longer Fair Value	2,231.5	3,309.1
12 Months or Longer Unrealized Losses	37.0	61.1
Total Fair Value	10,719.2	10,410.8
Total Unrealized Losses	62.8	87.6
U.S. Government
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value		998.2
Less than 12 Months Unrealized Losses		1.7
Total Fair Value		998.2
Total Unrealized Losses		1.7
U.S. Government Sponsored Agency
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	3,285.0	2,344.9
Less than 12 Months Unrealized Losses	8.2	6.6
12 Months or Longer Fair Value	914.3	1,730.0
12 Months or Longer Unrealized Losses	7.9	12.9
Total Fair Value	4,199.3	4,074.9
Total Unrealized Losses	16.1	19.5
Non-U.S. Government
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	212.7	292.9
Less than 12 Months Unrealized Losses	0.4	0.4
Total Fair Value	212.7	292.9
Total Unrealized Losses	0.4	0.4
Corporate Debt
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	943.8	1,244.5
Less than 12 Months Unrealized Losses	3.8	3.9
12 Months or Longer Fair Value	1,087.0	1,338.8
12 Months or Longer Unrealized Losses	15.3	37.7
Total Fair Value	2,030.8	2,583.3
Total Unrealized Losses	19.1	41.6
Covered Bonds
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	154.3	142.3
Less than 12 Months Unrealized Losses	0.2	0.2
12 Months or Longer Fair Value		10.0
12 Months or Longer Unrealized Losses		0.1
Total Fair Value	154.3	152.3
Total Unrealized Losses	0.2	0.3
Supranational and Non-U.S. Agency Bonds
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value		313.2
Less than 12 Months Unrealized Losses		0.3
12 Months or Longer Fair Value		175.5
12 Months or Longer Unrealized Losses		1.1
Total Fair Value		488.7
Total Unrealized Losses		1.4
Residential Mortgage-Backed
Investments, Unrealized Loss Position [Line Items]
12 Months or Longer Fair Value		4.5
12 Months or Longer Unrealized Losses		0.5
Total Fair Value		4.5
Total Unrealized Losses		0.5
Other Asset-Backed
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	1,233.7	1,297.6
Less than 12 Months Unrealized Losses	0.5	1.0
Total Fair Value	1,233.7	1,297.6
Total Unrealized Losses	0.5	1.0
Certificates of Deposit
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	1,878.5	438.6
Less than 12 Months Unrealized Losses	0.6	0.1
Total Fair Value	1,878.5	438.6
Total Unrealized Losses	0.6	0.1
Auction Rate Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	1.5	2.4
Less than 12 Months Unrealized Losses	0.1	0.2
12 Months or Longer Fair Value	4.6	4.7
12 Months or Longer Unrealized Losses	0.5	0.6
Total Fair Value	6.1	7.1
Total Unrealized Losses	0.6	0.8
Others
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	16.2	27.1
Less than 12 Months Unrealized Losses	10.8	12.1
12 Months or Longer Fair Value	48.2	45.6
12 Months or Longer Unrealized Losses	13.0	8.2
Total Fair Value	64.4	72.7
Total Unrealized Losses	23.8	$ 20.3
Sub-Sovereign, Supranational and Non US Agency Bonds
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	709.5
Less than 12 Months Unrealized Losses	1.0
12 Months or Longer Fair Value	177.4
12 Months or Longer Unrealized Losses	0.3
Total Fair Value	886.9
Total Unrealized Losses	1.3
Commercial Mortgage-Backed
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	52.5
Less than 12 Months Unrealized Losses	0.2
Total Fair Value	52.5
Total Unrealized Losses	$ 0.2