Securities Continuous Unrealized Loss Position (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	$ 18,412.8	$ 20,930.9
Less than 12 Months Unrealized Losses	130.2	85.3
12 Months or Longer Fair Value	3,627.4	2,617.7
12 Months or Longer Unrealized Losses	45.8	55.2
Total Fair Value	22,040.2	23,548.6
Total Unrealized Losses	176.0	140.5
U.S. Government Obligations
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	1,603.0	3,888.0
Less than 12 Months Unrealized Losses	14.3	5.5
12 Months or Longer Fair Value	0.0	0.0
12 Months or Longer Unrealized Losses	0.0	0.0
Total Fair Value	1,603.0	3,888.0
Total Unrealized Losses	14.3	5.5
Obligations of States and Political Subdivisions
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	865.3	15.1
Less than 12 Months Unrealized Losses	5.6	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	865.3	15.1
Total Unrealized Losses	5.6	0.1
Government Sponsored Agency
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	8,252.5	9,208.5
Less than 12 Months Unrealized Losses	58.5	38.7
12 Months or Longer Fair Value	2,121.0	1,213.6
12 Months or Longer Unrealized Losses	12.1	7.8
Total Fair Value	10,373.5	10,422.1
Total Unrealized Losses	70.6	46.5
Non-U.S. Government
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	2,957.1	314.3
Less than 12 Months Unrealized Losses	4.4	0.6
12 Months or Longer Fair Value	0.0	0.0
12 Months or Longer Unrealized Losses	0.0	0.0
Total Fair Value	2,957.1	314.3
Total Unrealized Losses	4.4	0.6
Corporate Debt
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	1,601.7	2,067.6
Less than 12 Months Unrealized Losses	11.2	10.3
12 Months or Longer Fair Value	1,054.4	1,057.1
12 Months or Longer Unrealized Losses	14.0	22.8
Total Fair Value	2,656.1	3,124.7
Total Unrealized Losses	25.2	33.1
Covered Bonds
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	809.0	1,598.4
Less than 12 Months Unrealized Losses	8.6	6.7
12 Months or Longer Fair Value	138.9	10.0
12 Months or Longer Unrealized Losses	0.6	0.1
Total Fair Value	947.9	1,608.4
Total Unrealized Losses	9.2	6.8
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	1,136.1	1,132.9
Less than 12 Months Unrealized Losses	5.7	2.3
12 Months or Longer Fair Value	249.1	109.3
12 Months or Longer Unrealized Losses	0.9	0.6
Total Fair Value	1,385.2	1,242.2
Total Unrealized Losses	6.6	2.9
Other Asset-Backed
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	584.3	2,122.7
Less than 12 Months Unrealized Losses	1.3	4.0
12 Months or Longer Fair Value	0.0	170.6
12 Months or Longer Unrealized Losses	0.0	0.2
Total Fair Value	584.3	2,293.3
Total Unrealized Losses	1.3	4.2
Certificates of Deposit
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	81.4	180.3
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	81.4	180.3
Total Unrealized Losses	0.1	0.1
Auction Rate
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	0.4	0.0
Less than 12 Months Unrealized Losses	0.1	0.0
12 Months or Longer Fair Value	4.3	6.4
12 Months or Longer Unrealized Losses	0.2	0.1
Total Fair Value	4.7	6.4
Total Unrealized Losses	0.3	0.1
Commercial Mortgage-Backed
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	471.5	374.4
Less than 12 Months Unrealized Losses	2.5	3.7
12 Months or Longer Fair Value	0.0	0.0
12 Months or Longer Unrealized Losses	0.0	0.0
Total Fair Value	471.5	374.4
Total Unrealized Losses	2.5	3.7
Other
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	50.5	28.7
Less than 12 Months Unrealized Losses	17.9	13.3
12 Months or Longer Fair Value	59.7	50.7
12 Months or Longer Unrealized Losses	18.0	23.6
Total Fair Value	110.2	79.4
Total Unrealized Losses	$ 35.9	$ 36.9