Securities (Securities with Unrealized Losses) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Fair Value
Less than 12 months, fair value	$ 20,766.1	$ 22,981.2
12 months or longer fair value	13,432.7	4,580.8
Total fair value	34,198.8	27,562.0
Unrealized Losses
Less than 12 months, unrealized losses	145.1	186.9
12 months or longer unrealized losses	285.4	62.5
Total unrealized losses	430.5	249.4
U.S. Government
Fair Value
Less than 12 months, fair value	1,459.1	3,595.0
12 months or longer fair value	2,167.6	0.0
Total fair value	3,626.7	3,595.0
Unrealized Losses
Less than 12 months, unrealized losses	22.2	32.1
12 months or longer unrealized losses	36.6	0.0
Total unrealized losses	58.8	32.1
Obligations of States and Political Subdivisions
Fair Value
Less than 12 months, fair value	471.5	687.8
12 months or longer fair value	241.0	52.0
Total fair value	712.5	739.8
Unrealized Losses
Less than 12 months, unrealized losses	5.0	3.3
12 months or longer unrealized losses	1.6	0.3
Total unrealized losses	6.6	3.6
Government Sponsored Agency
Fair Value
Less than 12 months, fair value	9,305.8	6,495.6
12 months or longer fair value	5,996.1	2,998.9
Total fair value	15,301.9	9,494.5
Unrealized Losses
Less than 12 months, unrealized losses	64.7	81.3
12 months or longer unrealized losses	134.9	27.3
Total unrealized losses	199.6	108.6
Non-U.S. Government
Fair Value
Less than 12 months, fair value	4,085.9	5,181.8
12 months or longer fair value	1,298.3	0.0
Total fair value	5,384.2	5,181.8
Unrealized Losses
Less than 12 months, unrealized losses	0.3	7.9
12 months or longer unrealized losses	14.0	0.0
Total unrealized losses	14.3	7.9
Corporate Debt
Fair Value
Less than 12 months, fair value	716.8	1,547.3
12 months or longer fair value	989.4	922.3
Total fair value	1,706.2	2,469.6
Unrealized Losses
Less than 12 months, unrealized losses	4.0	9.3
12 months or longer unrealized losses	23.7	14.0
Total unrealized losses	27.7	23.3
Covered Bonds
Fair Value
Less than 12 months, fair value	1,193.4	967.5
12 months or longer fair value	457.7	89.1
Total fair value	1,651.1	1,056.6
Unrealized Losses
Less than 12 months, unrealized losses	9.0	7.2
12 months or longer unrealized losses	9.3	0.9
Total unrealized losses	18.3	8.1
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Fair Value
Less than 12 months, fair value	1,617.9	1,692.4
12 months or longer fair value	1,244.4	235.8
Total fair value	2,862.3	1,928.2
Unrealized Losses
Less than 12 months, unrealized losses	11.1	7.5
12 months or longer unrealized losses	13.6	0.4
Total unrealized losses	24.7	7.9
Other Asset-Backed
Fair Value
Less than 12 months, fair value	1,787.6	2,453.7
12 months or longer fair value	606.3	29.9
Total fair value	2,393.9	2,483.6
Unrealized Losses
Less than 12 months, unrealized losses	8.1	8.3
12 months or longer unrealized losses	9.0	0.1
Total unrealized losses	17.1	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	78.3	233.5
12 months or longer fair value	326.3	201.6
Total fair value	404.6	435.1
Unrealized Losses
Less than 12 months, unrealized losses	1.0	2.6
12 months or longer unrealized losses	5.3	1.5
Total unrealized losses	6.3	4.1
Other
Fair Value
Less than 12 months, fair value	49.8	82.9
12 months or longer fair value	105.6	48.1
Total fair value	155.4	131.0
Unrealized Losses
Less than 12 months, unrealized losses	19.7	27.3
12 months or longer unrealized losses	37.4	17.9
Total unrealized losses	$ 57.1	$ 45.2