Investment Securities (Mortgage-backed Asset Securities) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investment securities
Fair value investments	$ 2,328.7	$ 3,208.3	$ 2,547.3
FNMA
Investment securities
Fair value investments	53.6	84.5
FHLMC
Investment securities
Fair value investments	38.1	62.0
GNMA
Investment securities
Fair value investments	23.7	46.3
Total Agency
Investment securities
Fair value investments	115.4	192.8
Commercial
Investment securities
Fair value investments	0.0	70.5
Total non-agency
Investment securities
Fair value investments	0.0	70.5
Total mortgage-backed securities
Investment securities
Fair value investments	115.4	263.3
Vehicle receivables
Investment securities
Fair value investments	9.2	142.4
Credit card receivables
Investment securities
Fair value investments	8.9	206.0
Other
Investment securities
Fair value investments	0.0	83.0
Total other asset-backed securities
Investment securities
Fair value investments	18.1	431.4
Total mortgage and asset- backed securities
Investment securities
Fair value investments	133.5	694.7
Level 2 Inputs
Investment securities
Fair value investments	1,160.5	2,105.4	$ 2,093.8
Level 2 Inputs | FNMA
Investment securities
Fair value investments	53.6	84.5
Level 2 Inputs | FHLMC
Investment securities
Fair value investments	38.1	62.0
Level 2 Inputs | GNMA
Investment securities
Fair value investments	23.7	46.3
Level 2 Inputs | Total Agency
Investment securities
Fair value investments	115.4	192.8
Level 2 Inputs | Commercial
Investment securities
Fair value investments	0.0	70.5
Level 2 Inputs | Total non-agency
Investment securities
Fair value investments	0.0	70.5
Level 2 Inputs | Total mortgage-backed securities
Investment securities
Fair value investments	115.4	263.3
Level 2 Inputs | Vehicle receivables
Investment securities
Fair value investments	9.2	142.4
Level 2 Inputs | Credit card receivables
Investment securities
Fair value investments	8.9	206.0
Level 2 Inputs | Other
Investment securities
Fair value investments	0.0	83.0
Level 2 Inputs | Total other asset-backed securities
Investment securities
Fair value investments	18.1	431.4
Level 2 Inputs | Total mortgage and asset- backed securities
Investment securities
Fair value investments	133.5	694.7
Level 3 Inputs | FNMA
Investment securities
Fair value investments	0.0	0.0
Level 3 Inputs | FHLMC
Investment securities
Fair value investments	0.0	0.0
Level 3 Inputs | GNMA
Investment securities
Fair value investments	0.0	0.0
Level 3 Inputs | Total Agency
Investment securities
Fair value investments	0.0	0.0
Level 3 Inputs | Commercial
Investment securities
Fair value investments	0.0	0.0
Level 3 Inputs | Total non-agency
Investment securities
Fair value investments	0.0	0.0
Level 3 Inputs | Total mortgage-backed securities
Investment securities
Fair value investments	0.0	0.0
Level 3 Inputs | Vehicle receivables
Investment securities
Fair value investments	0.0	0.0
Level 3 Inputs | Credit card receivables
Investment securities
Fair value investments	0.0	0.0
Level 3 Inputs | Other
Investment securities
Fair value investments	0.0	0.0
Level 3 Inputs | Total other asset-backed securities
Investment securities
Fair value investments	0.0	0.0
Level 3 Inputs | Total mortgage and asset- backed securities
Investment securities
Fair value investments	$ 0.0	$ 0.0