Investment Securities (Mortgage-backed Asset Securities) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Investment securities
Fair value investments	$ 5,039.7	$ 4,050.9	$ 3,343.6
GNMA
Investment securities
Fair value investments	283.9	265.5
FNMA
Investment securities
Fair value investments	278.3	42.2
FHLMC
Investment securities
Fair value investments	89.8	22.8
Total Agency
Investment securities
Fair value investments	652.0	330.5
Residential
Investment securities
Debt Instrument, Collateral Amount	30.2
Fair value investments	205.3	133.2
Commercial
Investment securities
Fair value investments	127.5	140.4
Total Non-agency
Investment securities
Fair value investments	332.8	273.6
Total mortgage-backed securities
Investment securities
Fair value investments	984.8	604.1
Credit card receivables
Investment securities
Fair value investments	438.3	217.7
Vehicle receivables
Investment securities
Fair value investments	479.5	269.7
Other
Investment securities
Fair value investments	230.3	75.5
Total other asset-backed securities
Investment securities
Fair value investments	1,148.1	562.9
Total mortgage and asset- backed securities
Investment securities
Fair value investments	2,132.9	1,167.0
Level 2 Inputs
Investment securities
Fair value investments	4,118.5	2,531.4	$ 2,372.9
Level 2 Inputs | GNMA
Investment securities
Fair value investments	283.9	265.5
Level 2 Inputs | FNMA
Investment securities
Fair value investments	278.3	42.2
Level 2 Inputs | FHLMC
Investment securities
Fair value investments	89.8	22.8
Level 2 Inputs | Total Agency
Investment securities
Fair value investments	652.0	330.5
Level 2 Inputs | Residential
Investment securities
Fair value investments	205.3	133.2
Level 2 Inputs | Commercial
Investment securities
Fair value investments	127.5	140.4
Level 2 Inputs | Total Non-agency
Investment securities
Fair value investments	332.8	273.6
Level 2 Inputs | Total mortgage-backed securities
Investment securities
Fair value investments	984.8	604.1
Level 2 Inputs | Credit card receivables
Investment securities
Fair value investments	438.3	217.7
Level 2 Inputs | Vehicle receivables
Investment securities
Fair value investments	479.5	269.7
Level 2 Inputs | Other
Investment securities
Fair value investments	230.3	75.5
Level 2 Inputs | Total other asset-backed securities
Investment securities
Fair value investments	1,148.1	562.9
Level 2 Inputs | Total mortgage and asset- backed securities
Investment securities
Fair value investments	2,132.9	1,167.0
Level 3 Inputs | GNMA
Investment securities
Fair value investments	0.0	0.0
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 | Total Agency
Investment securities
Fair value investments	0.0	0.0
Level 3 Inputs | Residential
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 | Credit card receivables
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 | 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