Securities Continuous Unrealized Loss Position (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	$ 4,972.7	$ 7,101.7
Less than 12 Months Unrealized Losses	16.5	26.5
12 Months or Longer Fair Value	2,538.4	3,309.1
12 Months or Longer Unrealized Losses	32.7	61.1
Total Fair Value	7,511.1	10,410.8
Total Unrealized Losses	49.2	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	1,946.0	2,344.9
Less than 12 Months Unrealized Losses	3.9	6.6
12 Months or Longer Fair Value	1,091.7	1,730.0
12 Months or Longer Unrealized Losses	6.4	12.9
Total Fair Value	3,037.7	4,074.9
Total Unrealized Losses	10.3	19.5
Non-U.S. Government
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value		292.9
Less than 12 Months Unrealized Losses		0.4
Total Fair Value		292.9
Total Unrealized Losses		0.4
Corporate Debt
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	391.4	1,244.5
Less than 12 Months Unrealized Losses	0.6	3.9
12 Months or Longer Fair Value	1,217.8	1,338.8
12 Months or Longer Unrealized Losses	16.3	37.7
Total Fair Value	1,609.2	2,583.3
Total Unrealized Losses	16.9	41.6
Covered Bonds
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value		142.3
Less than 12 Months Unrealized Losses		0.2
12 Months or Longer Fair Value		10.0
12 Months or Longer Unrealized Losses		0.1
Total Fair Value		152.3
Total Unrealized Losses		0.3
Sub-Sovereign, Supranational and Non US Agency Bonds
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	149.8
Less than 12 Months Unrealized Losses	0.2
12 Months or Longer Fair Value	170.7
12 Months or Longer Unrealized Losses	0.4
Total Fair Value	320.5
Total Unrealized Losses	0.6
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	4.4	4.5
12 Months or Longer Unrealized Losses	0.5	0.5
Total Fair Value	4.4	4.5
Total Unrealized Losses	0.5	0.5
Other Asset-Backed
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	1,071.1	1,297.6
Less than 12 Months Unrealized Losses	0.5	1.0
Total Fair Value	1,071.1	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	1,395.3	438.6
Less than 12 Months Unrealized Losses	0.3	0.1
Total Fair Value	1,395.3	438.6
Total Unrealized Losses	0.3	0.1
Auction Rate Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	1.4	2.4
Less than 12 Months Unrealized Losses	0.2	0.2
12 Months or Longer Fair Value	5.5	4.7
12 Months or Longer Unrealized Losses	0.9	0.6
Total Fair Value	6.9	7.1
Total Unrealized Losses	1.1	0.8
Others
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	17.7	27.1
Less than 12 Months Unrealized Losses	10.8	12.1
12 Months or Longer Fair Value	48.3	45.6
12 Months or Longer Unrealized Losses	8.2	8.2
Total Fair Value	66.0	72.7
Total Unrealized Losses	$ 19.0	$ 20.3