Investment Securities - Mortgage-backed Asset Securities (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Total mortgage-backed securities
Investment securities
Fair value investments	$ 231.9	$ 256.5
Total mortgage-backed securities | Level 2
Investment securities
Fair value investments	231.9	256.5
Total mortgage-backed securities | Level 3
Investment securities
Fair value investments	0.0	0.0
Total mortgage-backed securities | FNMA
Investment securities
Fair value investments	124.5	125.4
Total mortgage-backed securities | FNMA | Level 2
Investment securities
Fair value investments	124.5	125.4
Total mortgage-backed securities | FNMA | Level 3
Investment securities
Fair value investments	0.0	0.0
Total mortgage-backed securities | FHLMC
Investment securities
Fair value investments	78.8	90.5
Total mortgage-backed securities | FHLMC | Level 2
Investment securities
Fair value investments	78.8	90.5
Total mortgage-backed securities | FHLMC | Level 3
Investment securities
Fair value investments	0.0	0.0
Total mortgage-backed securities | GNMA
Investment securities
Fair value investments	28.3	40.1
Total mortgage-backed securities | GNMA | Level 2
Investment securities
Fair value investments	28.3	40.1
Total mortgage-backed securities | GNMA | Level 3
Investment securities
Fair value investments	0.0	0.0
Total mortgage-backed securities | Total agency
Investment securities
Fair value investments	231.6	256.0
Total mortgage-backed securities | Total agency | Level 2
Investment securities
Fair value investments	231.6	256.0
Total mortgage-backed securities | Total agency | Level 3
Investment securities
Fair value investments	0.0	0.0
Total mortgage-backed securities | Non-agency: Residential
Investment securities
Fair value investments	0.3	0.5
Total mortgage-backed securities | Non-agency: Residential | Level 2
Investment securities
Fair value investments	0.3	0.5
Total mortgage-backed securities | Non-agency: Residential | Level 3
Investment securities
Fair value investments	0.0	0.0
Total mortgage-backed securities | Total non-agency
Investment securities
Fair value investments	0.3	0.5
Total mortgage-backed securities | Total non-agency | Level 2
Investment securities
Fair value investments	0.3	0.5
Total mortgage-backed securities | Total non-agency | Level 3
Investment securities
Fair value investments	0.0	0.0
Total other asset-backed securities
Investment securities
Fair value investments	22.3	21.1
Total other asset-backed securities | Level 2
Investment securities
Fair value investments	22.3	21.1
Total other asset-backed securities | Level 3
Investment securities
Fair value investments	0.0	0.0
Total other asset-backed securities | Credit card receivables
Investment securities
Fair value investments	11.9	12.3
Total other asset-backed securities | Credit card receivables | Level 2
Investment securities
Fair value investments	11.9	12.3
Total other asset-backed securities | Credit card receivables | Level 3
Investment securities
Fair value investments	0.0	0.0
Total other asset-backed securities | Vehicle receivables
Investment securities
Fair value investments	10.4	8.8
Total other asset-backed securities | Vehicle receivables | Level 2
Investment securities
Fair value investments	10.4	8.8
Total other asset-backed securities | Vehicle receivables | Level 3
Investment securities
Fair value investments	0.0	0.0
Mortgage and asset-backed securities
Investment securities
Fair value investments	254.2	277.6
Carrying Value	254.2	277.6
Mortgage and asset-backed securities | Level 2
Investment securities
Fair value investments	254.2	277.6
Mortgage and asset-backed securities | Level 3
Investment securities
Fair value investments	0.0	0.0
Collateralized loan obligations
Investment securities
Fair value investments	182.9	135.0
Carrying Value	182.9	135.0
Collateralized loan obligations | Level 2
Investment securities
Fair value investments	182.9	135.0
Collateralized loan obligations | Level 3
Investment securities
Fair value investments	0.0	0.0
Total mortgage and asset-backed securities and collateralized loan obligations
Investment securities
Fair value investments	437.1	412.6
Total mortgage and asset-backed securities and collateralized loan obligations | Level 2
Investment securities
Fair value investments	437.1	412.6
Total mortgage and asset-backed securities and collateralized loan obligations | Level 3
Investment securities
Fair value investments	$ 0.0	$ 0.0