Securities Continuous Unrealized Loss Position (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	$ 7,101.7	$ 9,473.9
Less than 12 Months Unrealized Losses	26.5	145.8
12 Months or Longer Fair Value	3,309.1	776.1
12 Months or Longer Unrealized Losses	61.1	34.6
Total Fair Value	10,410.8	10,250.0
Total Unrealized Losses	87.6	180.4
U.S. Government
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	998.2	896.4
Less than 12 Months Unrealized Losses	1.7	1.4
Total Fair Value	998.2	896.4
Total Unrealized Losses	1.7	1.4
U.S. Government Sponsored Agency
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	2,344.9	4,340.8
Less than 12 Months Unrealized Losses	6.6	42.6
12 Months or Longer Fair Value	1,730.0	413.7
12 Months or Longer Unrealized Losses	12.9	5.3
Total Fair Value	4,074.9	4,754.5
Total Unrealized Losses	19.5	47.9
Non-U.S. Government
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	292.9	176.7
Less than 12 Months Unrealized Losses	0.4	1.1
Total Fair Value	292.9	176.7
Total Unrealized Losses	0.4	1.1
Corporate Debt
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	1,244.5	1,759.5
Less than 12 Months Unrealized Losses	3.9	85.4
12 Months or Longer Fair Value	1,338.8	267.0
12 Months or Longer Unrealized Losses	37.7	15.8
Total Fair Value	2,583.3	2,026.5
Total Unrealized Losses	41.6	101.2
Covered Bonds
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	142.3	278.8
Less than 12 Months Unrealized Losses	0.2	5.7
12 Months or Longer Fair Value	10.0	9.9
12 Months or Longer Unrealized Losses	0.1	0.2
Total Fair Value	152.3	288.7
Total Unrealized Losses	0.3	5.9
Supranational and Non-U.S. Agency Bonds
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	313.2	612.7
Less than 12 Months Unrealized Losses	0.3	4.9
12 Months or Longer Fair Value	175.5
12 Months or Longer Unrealized Losses	1.1
Total Fair Value	488.7	612.7
Total Unrealized Losses	1.4	4.9
Residential Mortgage-Backed
Investments, Unrealized Loss Position [Line Items]
12 Months or Longer Fair Value	4.5	42.0
12 Months or Longer Unrealized Losses	0.5	4.4
Total Fair Value	4.5	42.0
Total Unrealized Losses	0.5	4.4
Other Asset-Backed
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	1,297.6	677.0
Less than 12 Months Unrealized Losses	1.0	0.4
Total Fair Value	1,297.6	677.0
Total Unrealized Losses	1.0	0.4
Certificates of Deposit
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	438.6	684.2
Less than 12 Months Unrealized Losses	0.1	0.2
Total Fair Value	438.6	684.2
Total Unrealized Losses	0.1	0.2
Auction Rate Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	2.4	22.1
Less than 12 Months Unrealized Losses	0.2	0.1
12 Months or Longer Fair Value	4.7	14.0
12 Months or Longer Unrealized Losses	0.6	0.7
Total Fair Value	7.1	36.1
Total Unrealized Losses	0.8	0.8
Others
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	27.1	25.7
Less than 12 Months Unrealized Losses	12.1	4.0
12 Months or Longer Fair Value	45.6	29.5
12 Months or Longer Unrealized Losses	8.2	8.2
Total Fair Value	72.7	55.2
Total Unrealized Losses	$ 20.3	$ 12.2