Securities - Securities With Unrealized Losses (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	$ 22,981.2	$ 18,412.8
Less than 12 Months Unrealized Losses	186.9	130.2
12 Months or Longer Fair Value	4,580.8	3,627.4
12 Months or Longer Unrealized Losses	62.5	45.8
Total Fair Value	27,562.0	22,040.2
Total Unrealized Losses	249.4	176.0
U.S. Government Obligations
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	3,595.0	1,603.0
Less than 12 Months Unrealized Losses	32.1	14.3
12 Months or Longer Fair Value	0.0	0.0
12 Months or Longer Unrealized Losses	0.0	0.0
Total Fair Value	3,595.0	1,603.0
Total Unrealized Losses		14.3
Obligations of States and Political Subdivisions
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	687.8	865.3
Less than 12 Months Unrealized Losses	3.3	5.6
12 Months or Longer Fair Value	52.0	0.0
12 Months or Longer Unrealized Losses	0.3	0.0
Total Fair Value	739.8	865.3
Total Unrealized Losses	3.6	5.6
Government Sponsored Agency
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	6,495.6	8,252.5
Less than 12 Months Unrealized Losses	81.3	58.5
12 Months or Longer Fair Value	2,998.9	2,121.0
12 Months or Longer Unrealized Losses	27.3	12.1
Total Fair Value	9,494.5	10,373.5
Total Unrealized Losses	108.6	70.6
Non-U.S. Government
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	5,181.8	2,957.1
Less than 12 Months Unrealized Losses	7.9	4.4
12 Months or Longer Fair Value	0.0	0.0
12 Months or Longer Unrealized Losses	0.0	0.0
Total Fair Value	5,181.8	2,957.1
Total Unrealized Losses	7.9	4.4
Corporate Debt
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	1,547.3	1,601.7
Less than 12 Months Unrealized Losses	9.3	11.2
12 Months or Longer Fair Value	922.3	1,054.4
12 Months or Longer Unrealized Losses	14.0	14.0
Total Fair Value	2,469.6	2,656.1
Total Unrealized Losses	23.3	25.2
Covered Bonds
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	967.5	809.0
Less than 12 Months Unrealized Losses	7.2	8.6
12 Months or Longer Fair Value	89.1	138.9
12 Months or Longer Unrealized Losses	0.9	0.6
Total Fair Value	1,056.6	947.9
Total Unrealized Losses	8.1	9.2
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	1,692.4	1,136.1
Less than 12 Months Unrealized Losses	7.5	5.7
12 Months or Longer Fair Value	235.8	249.1
12 Months or Longer Unrealized Losses	0.4	0.9
Total Fair Value	1,928.2	1,385.2
Total Unrealized Losses	7.9	6.6
Other Asset-Backed
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	2,453.7	584.3
Less than 12 Months Unrealized Losses	8.3	1.3
12 Months or Longer Fair Value	29.9	0.0
12 Months or Longer Unrealized Losses	0.1	0.0
Total Fair Value	2,483.6	584.3
Total Unrealized Losses	8.4	1.3
Certificates of Deposit
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	43.7	81.4
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	43.7	81.4
Total Unrealized Losses	0.1	0.1
Auction Rate
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	0.0	0.4
Less than 12 Months Unrealized Losses	0.0	0.1
12 Months or Longer Fair Value	3.1	4.3
12 Months or Longer Unrealized Losses	0.1	0.2
Total Fair Value	3.1	4.7
Total Unrealized Losses	0.1	0.3
Commercial Mortgage-Backed
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	233.5	471.5
Less than 12 Months Unrealized Losses	2.6	2.5
12 Months or Longer Fair Value	201.6	0.0
12 Months or Longer Unrealized Losses	1.5	0.0
Total Fair Value	435.1	471.5
Total Unrealized Losses	4.1	2.5
Other
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	82.9	50.5
Less than 12 Months Unrealized Losses	27.3	17.9
12 Months or Longer Fair Value	48.1	59.7
12 Months or Longer Unrealized Losses	17.9	18.0
Total Fair Value	131.0	110.2
Total Unrealized Losses	$ 45.2	$ 35.9