Investment Securities - Mortgage-backed Asset Securities (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Investment securities
Fair value investments	$ 2,796.2	$ 2,746.0	$ 2,328.7
FNMA
Investment securities
Fair value investments	88.7	88.6
FHLMC
Investment securities
Fair value investments	70.1	60.5
GNMA
Investment securities
Fair value investments	40.6	30.8
Total Agency
Investment securities
Fair value investments	199.4	179.9
Total mortgage-backed securities
Investment securities
Fair value investments	199.4	179.9
Credit card receivables
Investment securities
Fair value investments	11.3	14.0
Vehicle receivables
Investment securities
Fair value investments	7.8	15.1
Total other asset-backed securities
Investment securities
Fair value investments	19.1	29.1
Total mortgage and asset- backed securities
Investment securities
Fair value investments	218.5	209.0
Level 2
Investment securities
Fair value investments	1,030.9	1,109.6	$ 1,160.5
Level 2 | FNMA
Investment securities
Fair value investments	88.7	88.6
Level 2 | FHLMC
Investment securities
Fair value investments	70.1	60.5
Level 2 | GNMA
Investment securities
Fair value investments	40.6	30.8
Level 2 | Total Agency
Investment securities
Fair value investments	199.4	179.9
Level 2 | Total mortgage-backed securities
Investment securities
Fair value investments	199.4	179.9
Level 2 | Credit card receivables
Investment securities
Fair value investments	11.3	14.0
Level 2 | Vehicle receivables
Investment securities
Fair value investments	7.8	15.1
Level 2 | Total other asset-backed securities
Investment securities
Fair value investments	19.1	29.1
Level 2 | Total mortgage and asset- backed securities
Investment securities
Fair value investments	218.5	209.0
Level 3 | FNMA
Investment securities
Fair value investments	0.0	0.0
Level 3 | FHLMC
Investment securities
Fair value investments	0.0	0.0
Level 3 | GNMA
Investment securities
Fair value investments	0.0	0.0
Level 3 | Total Agency
Investment securities
Fair value investments	0.0	0.0
Level 3 | Total mortgage-backed securities
Investment securities
Fair value investments	0.0	0.0
Level 3 | Credit card receivables
Investment securities
Fair value investments	0.0	0.0
Level 3 | Vehicle receivables
Investment securities
Fair value investments	0.0	0.0
Level 3 | Total other asset-backed securities
Investment securities
Fair value investments	0.0	0.0
Level 3 | Total mortgage and asset- backed securities
Investment securities
Fair value investments	$ 0.0	$ 0.0