Securities - Securities With Unrealized Losses (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Fair Value
Less Than 12 Months, Fair Value	$ 13,362.9	$ 18,648.8
12 Months or Longer, Fair Value	10,754.4	15,563.4
Total, Fair Value	24,117.3	34,212.2
Unrealized Losses
Less Than 12 Months, Unrealized Losses	91.2	83.9
12 Months or Longer, Unrealized Losses	98.3	273.2
Total, Unrealized Losses	189.5	357.1
U.S. Government
Fair Value
Less Than 12 Months, Fair Value	252.2	0.0
12 Months or Longer, Fair Value	899.8	2,862.0
Total, Fair Value	1,152.0	2,862.0
Unrealized Losses
Less Than 12 Months, Unrealized Losses	2.7	0.0
12 Months or Longer, Unrealized Losses	2.4	39.6
Total, Unrealized Losses	5.1	39.6
Obligations of States and Political Subdivisions
Fair Value
Less Than 12 Months, Fair Value	902.5	169.6
12 Months or Longer, Fair Value	0.0	279.6
Total, Fair Value	902.5	449.2
Unrealized Losses
Less Than 12 Months, Unrealized Losses	13.3	2.4
12 Months or Longer, Unrealized Losses	0.0	1.3
Total, Unrealized Losses	13.3	3.7
Government Sponsored Agency
Fair Value
Less Than 12 Months, Fair Value	5,405.0	8,368.8
12 Months or Longer, Fair Value	7,818.4	6,822.4
Total, Fair Value	13,223.4	15,191.2
Unrealized Losses
Less Than 12 Months, Unrealized Losses	35.6	33.5
12 Months or Longer, Unrealized Losses	42.5	117.0
Total, Unrealized Losses	78.1	150.5
Non-U.S. Government
Fair Value
Less Than 12 Months, Fair Value	3,620.2	5,065.2
12 Months or Longer, Fair Value	0.0	1,274.0
Total, Fair Value	3,620.2	6,339.2
Unrealized Losses
Less Than 12 Months, Unrealized Losses	2.5	0.8
12 Months or Longer, Unrealized Losses	0.0	9.0
Total, Unrealized Losses	2.5	9.8
Corporate Debt
Fair Value
Less Than 12 Months, Fair Value	410.4	712.7
12 Months or Longer, Fair Value	492.8	1,097.4
Total, Fair Value	903.2	1,810.1
Unrealized Losses
Less Than 12 Months, Unrealized Losses	1.3	4.1
12 Months or Longer, Unrealized Losses	3.0	18.8
Total, Unrealized Losses	4.3	22.9
Covered Bonds
Fair Value
Less Than 12 Months, Fair Value	646.8	646.4
12 Months or Longer, Fair Value	0.0	696.9
Total, Fair Value	646.8	1,343.3
Unrealized Losses
Less Than 12 Months, Unrealized Losses	3.2	3.7
12 Months or Longer, Unrealized Losses	0.0	10.4
Total, Unrealized Losses	3.2	14.1
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Fair Value
Less Than 12 Months, Fair Value	1,302.0	1,105.0
12 Months or Longer, Fair Value	155.2	1,189.2
Total, Fair Value	1,457.2	2,294.2
Unrealized Losses
Less Than 12 Months, Unrealized Losses	3.1	4.6
12 Months or Longer, Unrealized Losses	0.1	11.2
Total, Unrealized Losses	3.2	15.8
Other Asset-Backed
Fair Value
Less Than 12 Months, Fair Value	706.9	2,507.8
12 Months or Longer, Fair Value	1,164.9	954.9
Total, Fair Value	1,871.8	3,462.7
Unrealized Losses
Less Than 12 Months, Unrealized Losses	2.1	15.9
12 Months or Longer, Unrealized Losses	3.5	11.0
Total, Unrealized Losses	5.6	26.9
Commercial Mortgage-Backed
Fair Value
Less Than 12 Months, Fair Value	62.8	22.8
12 Months or Longer, Fair Value	59.3	274.4
Total, Fair Value	122.1	297.2
Unrealized Losses
Less Than 12 Months, Unrealized Losses	0.7	0.1
12 Months or Longer, Unrealized Losses	0.2	4.1
Total, Unrealized Losses	0.9	4.2
Other
Fair Value
Less Than 12 Months, Fair Value	54.1	50.5
12 Months or Longer, Fair Value	164.0	112.6
Total, Fair Value	218.1	163.1
Unrealized Losses
Less Than 12 Months, Unrealized Losses	26.7	18.8
12 Months or Longer, Unrealized Losses	46.6	50.8
Total, Unrealized Losses	$ 73.3	$ 69.6