Securities - Securities With Unrealized Losses (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Fair Value
Less Than 12 Months, Fair Value	$ 18,648.8	$ 22,981.2
12 Months or Longer, Fair Value	15,563.4	4,580.8
Total, Fair Value	34,212.2	27,562.0
Unrealized Losses
Less Than 12 Months, Unrealized Losses	83.9	186.9
12 Months or Longer, Unrealized Losses	273.2	62.5
Total, Unrealized Losses	357.1	249.4
U.S. Government
Fair Value
Less Than 12 Months, Fair Value	0.0	3,595.0
12 Months or Longer, Fair Value	2,862.0	0.0
Total, Fair Value	2,862.0	3,595.0
Unrealized Losses
Less Than 12 Months, Unrealized Losses	0.0	32.1
12 Months or Longer, Unrealized Losses	39.6	0.0
Total, Unrealized Losses	39.6	32.1
Obligations of States and Political Subdivisions
Fair Value
Less Than 12 Months, Fair Value	169.6	687.8
12 Months or Longer, Fair Value	279.6	52.0
Total, Fair Value	449.2	739.8
Unrealized Losses
Less Than 12 Months, Unrealized Losses	2.4	3.3
12 Months or Longer, Unrealized Losses	1.3	0.3
Total, Unrealized Losses	3.7	3.6
Government Sponsored Agency
Fair Value
Less Than 12 Months, Fair Value	8,368.8	6,495.6
12 Months or Longer, Fair Value	6,822.4	2,998.9
Total, Fair Value	15,191.2	9,494.5
Unrealized Losses
Less Than 12 Months, Unrealized Losses	33.5	81.3
12 Months or Longer, Unrealized Losses	117.0	27.3
Total, Unrealized Losses	150.5	108.6
Non-U.S. Government
Fair Value
Less Than 12 Months, Fair Value	5,065.2	5,181.8
12 Months or Longer, Fair Value	1,274.0	0.0
Total, Fair Value	6,339.2	5,181.8
Unrealized Losses
Less Than 12 Months, Unrealized Losses	0.8	7.9
12 Months or Longer, Unrealized Losses	9.0	0.0
Total, Unrealized Losses	9.8	7.9
Corporate Debt
Fair Value
Less Than 12 Months, Fair Value	712.7	1,547.3
12 Months or Longer, Fair Value	1,097.4	922.3
Total, Fair Value	1,810.1	2,469.6
Unrealized Losses
Less Than 12 Months, Unrealized Losses	4.1	9.3
12 Months or Longer, Unrealized Losses	18.8	14.0
Total, Unrealized Losses	22.9	23.3
Covered Bonds
Fair Value
Less Than 12 Months, Fair Value	646.4	967.5
12 Months or Longer, Fair Value	696.9	89.1
Total, Fair Value	1,343.3	1,056.6
Unrealized Losses
Less Than 12 Months, Unrealized Losses	3.7	7.2
12 Months or Longer, Unrealized Losses	10.4	0.9
Total, Unrealized Losses	14.1	8.1
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Fair Value
Less Than 12 Months, Fair Value	1,105.0	1,692.4
12 Months or Longer, Fair Value	1,189.2	235.8
Total, Fair Value	2,294.2	1,928.2
Unrealized Losses
Less Than 12 Months, Unrealized Losses	4.6	7.5
12 Months or Longer, Unrealized Losses	11.2	0.4
Total, Unrealized Losses	15.8	7.9
Other Asset-Backed
Fair Value
Less Than 12 Months, Fair Value	2,507.8	2,453.7
12 Months or Longer, Fair Value	954.9	29.9
Total, Fair Value	3,462.7	2,483.6
Unrealized Losses
Less Than 12 Months, Unrealized Losses	15.9	8.3
12 Months or Longer, Unrealized Losses	11.0	0.1
Total, Unrealized Losses	26.9	8.4
Certificates of Deposit
Fair Value
Less Than 12 Months, Fair Value		43.7
12 Months or Longer, Fair Value		0.0
Total, Fair Value		43.7
Unrealized Losses
Less Than 12 Months, Unrealized Losses		0.1
12 Months or Longer, Unrealized Losses		0.0
Total, Unrealized Losses		0.1
Auction Rate
Fair Value
Less Than 12 Months, Fair Value		0.0
12 Months or Longer, Fair Value		3.1
Total, Fair Value		3.1
Unrealized Losses
Less Than 12 Months, Unrealized Losses		0.0
12 Months or Longer, Unrealized Losses		0.1
Total, Unrealized Losses		0.1
Commercial Mortgage-Backed
Fair Value
Less Than 12 Months, Fair Value	22.8	233.5
12 Months or Longer, Fair Value	274.4	201.6
Total, Fair Value	297.2	435.1
Unrealized Losses
Less Than 12 Months, Unrealized Losses	0.1	2.6
12 Months or Longer, Unrealized Losses	4.1	1.5
Total, Unrealized Losses	4.2	4.1
Other
Fair Value
Less Than 12 Months, Fair Value	50.5	82.9
12 Months or Longer, Fair Value	112.6	48.1
Total, Fair Value	163.1	131.0
Unrealized Losses
Less Than 12 Months, Unrealized Losses	18.8	27.3
12 Months or Longer, Unrealized Losses	50.8	17.9
Total, Unrealized Losses	$ 69.6	$ 45.2