Investment Securities (Mortgage-backed Asset Securities) (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Investment securities
Fair value investments	$ 4,050.9	$ 3,343.6	$ 3,889.0
GNMA
Investment securities
Fair value investments	265.5	308.7
FNMA
Investment securities
Fair value investments	42.2	33.4
FHLMC
Investment securities
Fair value investments	22.8	28.9
Total Agency
Investment securities
Fair value investments	330.5	371.0
Residential
Investment securities
Fair value investments	133.2	71.0
Commercial
Investment securities
Fair value investments	140.4	109.3
Total Non-agency
Investment securities
Fair value investments	273.6	180.3
Total mortgage-backed securities
Investment securities
Fair value investments	604.1	551.3
Credit card receivables
Investment securities
Fair value investments	217.7	218.1
Vehicle receivables
Investment securities
Fair value investments	269.7	152.8
Other
Investment securities
Fair value investments	75.5	45.7
Total asset-backed securities
Investment securities
Fair value investments	562.9	416.6
Total mortgage and asset- backed securities
Investment securities
Fair value investments	1,167.0	967.9
Non-Agency Residential Mortgage
Investment securities
Fair value investments	39.6
Level 2 Inputs
Investment securities
Fair value investments	2,531.4	2,372.9	$ 2,575.4
Level 2 Inputs | GNMA
Investment securities
Fair value investments	265.5	308.7
Level 2 Inputs | FNMA
Investment securities
Fair value investments	42.2	33.4
Level 2 Inputs | FHLMC
Investment securities
Fair value investments	22.8	28.9
Level 2 Inputs | Total Agency
Investment securities
Fair value investments	330.5	371.0
Level 2 Inputs | Residential
Investment securities
Fair value investments	133.2	71.0
Level 2 Inputs | Commercial
Investment securities
Fair value investments	140.4	109.3
Level 2 Inputs | Total Non-agency
Investment securities
Fair value investments	273.6	180.3
Level 2 Inputs | Total mortgage-backed securities
Investment securities
Fair value investments	604.1	551.3
Level 2 Inputs | Credit card receivables
Investment securities
Fair value investments	217.7	218.1
Level 2 Inputs | Vehicle receivables
Investment securities
Fair value investments	269.7	152.8
Level 2 Inputs | Other
Investment securities
Fair value investments	75.5	45.7
Level 2 Inputs | Total asset-backed securities
Investment securities
Fair value investments	562.9	416.6
Level 2 Inputs | Total mortgage and asset- backed securities
Investment securities
Fair value investments	1,167.0	967.9
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 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