Investment Securities (Mortgage-backed Asset Securities) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investment securities
Fair value investments	$ 3,208.3	$ 2,547.3	$ 1,543.5
GNMA
Investment securities
Fair value investments	46.3	70.3
FNMA
Investment securities
Fair value investments	84.5	235.5
FHLMC
Investment securities
Fair value investments	62.0	59.5
Total Agency
Investment securities
Fair value investments	192.8	365.3
Residential
Investment securities
Fair value investments	0.0	70.3
Commercial
Investment securities
Fair value investments	70.5	3.9
Total Non-agency
Investment securities
Fair value investments	70.5	74.2
Total mortgage-backed securities
Investment securities
Fair value investments	263.3	439.5
Credit card receivables
Investment securities
Fair value investments	206.0	214.2
Vehicle receivables
Investment securities
Fair value investments	142.4	205.9
Other
Investment securities
Fair value investments	83.0	120.2
Total other asset-backed securities
Investment securities
Fair value investments	431.4	540.3
Total mortgage and asset- backed securities
Investment securities
Fair value investments	694.7	979.8
Level 2 Inputs
Investment securities
Fair value investments	2,105.4	2,093.8	$ 585.6
Level 2 Inputs | GNMA
Investment securities
Fair value investments	46.3	70.3
Level 2 Inputs | FNMA
Investment securities
Fair value investments	84.5	235.5
Level 2 Inputs | FHLMC
Investment securities
Fair value investments	62.0	59.5
Level 2 Inputs | Total Agency
Investment securities
Fair value investments	192.8	365.3
Level 2 Inputs | Residential
Investment securities
Fair value investments	0.0	70.3
Level 2 Inputs | Commercial
Investment securities
Fair value investments	70.5	3.9
Level 2 Inputs | Total Non-agency
Investment securities
Fair value investments	70.5	74.2
Level 2 Inputs | Total mortgage-backed securities
Investment securities
Fair value investments	263.3	439.5
Level 2 Inputs | Credit card receivables
Investment securities
Fair value investments	206.0	214.2
Level 2 Inputs | Vehicle receivables
Investment securities
Fair value investments	142.4	205.9
Level 2 Inputs | Other
Investment securities
Fair value investments	83.0	120.2
Level 2 Inputs | Total other asset-backed securities
Investment securities
Fair value investments	431.4	540.3
Level 2 Inputs | Total mortgage and asset- backed securities
Investment securities
Fair value investments	694.7	979.8
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