Securities Continuous Unrealized Loss Position (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	$ 6,359.4	$ 9,473.9
Less than 12 Months Unrealized Losses	22.3	145.8
12 Months or Longer Fair Value	3,493.1	776.1
12 Months or Longer Unrealized Losses	60.2	34.6
Total Fair Value	9,852.5	10,250.0
Total Unrealized Losses	82.5	180.4
U.S. Government
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	1,596.0	896.4
Less than 12 Months Unrealized Losses	1.5	1.4
Total Fair Value	1,596.0	896.4
Total Unrealized Losses	1.5	1.4
U.S. Government Sponsored Agency
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	2,099.2	4,340.8
Less than 12 Months Unrealized Losses	8.3	42.6
12 Months or Longer Fair Value	1,773.5	413.7
12 Months or Longer Unrealized Losses	16.9	5.3
Total Fair Value	3,872.7	4,754.5
Total Unrealized Losses	25.2	47.9
Non-U.S. Government
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	235.4	176.7
Less than 12 Months Unrealized Losses	0.1	1.1
12 Months or Longer Fair Value	80.3
12 Months or Longer Unrealized Losses	0.6
Total Fair Value	315.7	176.7
Total Unrealized Losses	0.7	1.1
Corporate Debt
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	862.3	1,759.5
Less than 12 Months Unrealized Losses	2.8	85.4
12 Months or Longer Fair Value	1,258.0	267.0
12 Months or Longer Unrealized Losses	30.6	15.8
Total Fair Value	2,120.3	2,026.5
Total Unrealized Losses	33.4	101.2
Covered Bonds
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	77.0	278.8
Less than 12 Months Unrealized Losses	0.1	5.7
12 Months or Longer Fair Value	9.9	9.9
12 Months or Longer Unrealized Losses	0.1	0.2
Total Fair Value	86.9	288.7
Total Unrealized Losses	0.2	5.9
Supranational and Non-U.S. Agency Bonds
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	438.2	612.7
Less than 12 Months Unrealized Losses	0.7	4.9
12 Months or Longer Fair Value	300.8
12 Months or Longer Unrealized Losses	2.4
Total Fair Value	739.0	612.7
Total Unrealized Losses	3.1	4.9
Residential Mortgage-Backed
Investments, Unrealized Loss Position [Line Items]
12 Months or Longer Fair Value	15.0	42.0
12 Months or Longer Unrealized Losses	1.3	4.4
Total Fair Value	15.0	42.0
Total Unrealized Losses	1.3	4.4
Other Asset-Backed
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	485.2	677.0
Less than 12 Months Unrealized Losses	0.1	0.4
12 Months or Longer Fair Value	7.1
12 Months or Longer Unrealized Losses	0.1
Total Fair Value	492.3	677.0
Total Unrealized Losses	0.2	0.4
Certificates of Deposit
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	552.0	684.2
Less than 12 Months Unrealized Losses	0.1	0.2
Total Fair Value	552.0	684.2
Total Unrealized Losses	0.1	0.2
Auction Rate Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	1.1	22.1
Less than 12 Months Unrealized Losses	0.1	0.1
12 Months or Longer Fair Value	5.1	14.0
12 Months or Longer Unrealized Losses	0.4	0.7
Total Fair Value	6.2	36.1
Total Unrealized Losses	0.5	0.8
Others
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	13.0	25.7
Less than 12 Months Unrealized Losses	8.5	4.0
12 Months or Longer Fair Value	43.4	29.5
12 Months or Longer Unrealized Losses	7.8	8.2
Total Fair Value	56.4	55.2
Total Unrealized Losses	$ 16.3	$ 12.2