Investment Securities (Mortgage-backed Asset Securities) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Investment securities
Fair value investments	$ 5,991.4	$ 6,766.5	$ 6,950.7
GNMA
Investment securities
Fair value investments	411.2	512.3
FNMA
Investment securities
Fair value investments	36.6	81.2
FHLMC
Investment securities
Fair value investments	49.6	91.3
Total Agency
Investment securities
Fair value investments	497.4	684.8
Residential
Investment securities
Fair value investments	131.2	125.7
Commercial
Investment securities
Fair value investments	236.9	282.3
Total Non-agency
Investment securities
Fair value investments	368.1	408.0
Total mortgage-backed securities
Investment securities
Fair value investments	865.5	1,092.8
Credit card receivables
Investment securities
Fair value investments	522.2	311.4
Vehicle receivables
Investment securities
Fair value investments	289.4	365.0
Other
Investment securities
Fair value investments	163.8	245.3
Total asset-backed securities
Investment securities
Fair value investments	975.4	921.7
Total mortgage and asset- backed securities
Investment securities
Fair value investments	1,840.9	2,014.5
Non-Agency Residential Mortgage
Investment securities
Fair value investments	45.1
Level 2 Inputs
Investment securities
Fair value investments	4,638.0	4,982.2	5,206.1
Level 2 Inputs | GNMA
Investment securities
Fair value investments	411.2	512.3
Level 2 Inputs | FNMA
Investment securities
Fair value investments	36.6	81.2
Level 2 Inputs | FHLMC
Investment securities
Fair value investments	49.6	91.3
Level 2 Inputs | Total Agency
Investment securities
Fair value investments	497.4	684.8
Level 2 Inputs | Residential
Investment securities
Fair value investments	131.2	125.7
Level 2 Inputs | Commercial
Investment securities
Fair value investments	236.9	282.3
Level 2 Inputs | Total Non-agency
Investment securities
Fair value investments	368.1	408.0
Level 2 Inputs | Total mortgage-backed securities
Investment securities
Fair value investments	865.5	1,092.8
Level 2 Inputs | Credit card receivables
Investment securities
Fair value investments	522.2	289.4
Level 2 Inputs | Vehicle receivables
Investment securities
Fair value investments	289.4	365.0
Level 2 Inputs | Other
Investment securities
Fair value investments	163.8	245.3
Level 2 Inputs | Total asset-backed securities
Investment securities
Fair value investments	975.4	899.7
Level 2 Inputs | Total mortgage and asset- backed securities
Investment securities
Fair value investments	1,840.9	1,992.5
Level 3 Inputs | GNMA
Investment securities
Fair value investments	0	0
Level 3 Inputs | FNMA
Investment securities
Fair value investments	0	0
Level 3 Inputs | FHLMC
Investment securities
Fair value investments	0	0
Level 3 Inputs | Total Agency
Investment securities
Fair value investments	0	0
Level 3 Inputs | Residential
Investment securities
Fair value investments	0	0
Level 3 Inputs | Commercial
Investment securities
Fair value investments	0	0
Level 3 Inputs | Total Non-agency
Investment securities
Fair value investments	0	0
Level 3 Inputs | Total mortgage-backed securities
Investment securities
Fair value investments	0	0
Level 3 Inputs | Credit card receivables
Investment securities
Fair value investments	0	22.0
Level 3 Inputs | Vehicle receivables
Investment securities
Fair value investments	0	0
Level 3 Inputs | Other
Investment securities
Fair value investments	0	0
Level 3 Inputs | Total asset-backed securities
Investment securities
Fair value investments	0	22.0
Level 3 Inputs | Total mortgage and asset- backed securities
Investment securities
Fair value investments	$ 0	$ 22.0