Securities Continuous Unrealized Loss Position (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	$ 20,930.9	$ 7,101.7
Less than 12 Months Unrealized Losses	85.3	26.5
12 Months or Longer Fair Value	2,617.7	3,309.1
12 Months or Longer Unrealized Losses	55.2	61.1
Total Fair Value	23,548.6	10,410.8
Total Unrealized Losses	140.5	87.6
U.S. Government
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	3,888.0	998.2
Less than 12 Months Unrealized Losses	5.5	1.7
12 Months or Longer Fair Value	0.0	0.0
12 Months or Longer Unrealized Losses	0.0	0.0
Total Fair Value	3,888.0	998.2
Total Unrealized Losses	5.5	1.7
Obligations of States and Political Subdivisions
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	15.1
Less than 12 Months Unrealized Losses	0.1
12 Months or Longer Fair Value	0.0
12 Months or Longer Unrealized Losses	0.0
Total Fair Value	15.1
Total Unrealized Losses	0.1
U.S. Government Sponsored Agency
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	9,208.5	2,344.9
Less than 12 Months Unrealized Losses	38.7	6.6
12 Months or Longer Fair Value	1,213.6	1,730.0
12 Months or Longer Unrealized Losses	7.8	12.9
Total Fair Value	10,422.1	4,074.9
Total Unrealized Losses	46.5	19.5
Non-U.S. Government
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	314.3	292.9
Less than 12 Months Unrealized Losses	0.6	0.4
12 Months or Longer Fair Value	0.0	0.0
12 Months or Longer Unrealized Losses	0.0	0.0
Total Fair Value	314.3	292.9
Total Unrealized Losses	0.6	0.4
Corporate Debt
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	2,067.6	1,244.5
Less than 12 Months Unrealized Losses	10.3	3.9
12 Months or Longer Fair Value	1,057.1	1,338.8
12 Months or Longer Unrealized Losses	22.8	37.7
Total Fair Value	3,124.7	2,583.3
Total Unrealized Losses	33.1	41.6
Covered Bonds
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	1,598.4	142.3
Less than 12 Months Unrealized Losses	6.7	0.2
12 Months or Longer Fair Value	10.0	10.0
12 Months or Longer Unrealized Losses	0.1	0.1
Total Fair Value	1,608.4	152.3
Total Unrealized Losses	6.8	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
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	1,132.9
Less than 12 Months Unrealized Losses	2.3
12 Months or Longer Fair Value	109.3
12 Months or Longer Unrealized Losses	0.6
Total Fair Value	1,242.2
Total Unrealized Losses	2.9
Residential Mortgage-Backed
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value		0.0
Less than 12 Months Unrealized Losses		0.0
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	2,122.7	1,297.6
Less than 12 Months Unrealized Losses	4.0	1.0
12 Months or Longer Fair Value	170.6	0.0
12 Months or Longer Unrealized Losses	0.2	0.0
Total Fair Value	2,293.3	1,297.6
Total Unrealized Losses	4.2	1.0
Certificates of Deposit
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	180.3	438.6
Less than 12 Months Unrealized Losses	0.1	0.1
12 Months or Longer Fair Value	0.0	0.0
12 Months or Longer Unrealized Losses	0.0	0.0
Total Fair Value	180.3	438.6
Total Unrealized Losses	0.1	0.1
Auction Rate Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	0.0	2.4
Less than 12 Months Unrealized Losses	0.0	0.2
12 Months or Longer Fair Value	6.4	4.7
12 Months or Longer Unrealized Losses	0.1	0.6
Total Fair Value	6.4	7.1
Total Unrealized Losses	0.1	0.8
Commercial Mortgage Backed Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	374.4
Less than 12 Months Unrealized Losses	3.7
12 Months or Longer Fair Value	0.0
12 Months or Longer Unrealized Losses	0.0
Total Fair Value	374.4
Total Unrealized Losses	3.7
Others
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	28.7	27.1
Less than 12 Months Unrealized Losses	13.3	12.1
12 Months or Longer Fair Value	50.7	45.6
12 Months or Longer Unrealized Losses	23.6	8.2
Total Fair Value	79.4	72.7
Total Unrealized Losses	$ 36.9	$ 20.3