Investment Securities - Mortgage-backed Asset Securities (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Total mortgage-backed securities
Investment securities
Fair value investments	$ 256.5	$ 199.4
Total mortgage-backed securities | Level 2
Investment securities
Fair value investments	256.5	199.4
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	125.4	88.7
Total mortgage-backed securities | FNMA | Level 2
Investment securities
Fair value investments	125.4	88.7
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	90.5	70.1
Total mortgage-backed securities | FHLMC | Level 2
Investment securities
Fair value investments	90.5	70.1
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	40.1	40.6
Total mortgage-backed securities | GNMA | Level 2
Investment securities
Fair value investments	40.1	40.6
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	256.0	199.4
Total mortgage-backed securities | Total agency | Level 2
Investment securities
Fair value investments	256.0	199.4
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	0.5	0.0
Total mortgage-backed securities | Non-agency: Residential | Level 2
Investment securities
Fair value investments	0.5	0.0
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	0.5	0.0
Total mortgage-backed securities | Total non-agency | Level 2
Investment securities
Fair value investments	0.5	0.0
Total mortgage-backed securities | Total non-agency | Level 3
Investment securities
Fair value investments	0.0	0.0
Total mortgage-backed securities | Collateralized mortgage obligations
Investment securities
Fair value investments	135.0
Total other asset-backed securities
Investment securities
Fair value investments	21.1	19.1
Total other asset-backed securities | Level 2
Investment securities
Fair value investments	21.1	19.1
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	12.3	11.3
Total other asset-backed securities | Credit card receivables | Level 2
Investment securities
Fair value investments	12.3	11.3
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	8.8	7.8
Total other asset-backed securities | Vehicle receivables | Level 2
Investment securities
Fair value investments	8.8	7.8
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	277.6	218.5
Mortgage and asset-backed securities | Level 2
Investment securities
Fair value investments	277.6	218.5
Mortgage and asset-backed securities | Level 3
Investment securities
Fair value investments	0.0	0.0
Collateralized loan obligations
Investment securities
Fair value investments	135.0	0.0
Collateralized loan obligations | Level 2
Investment securities
Fair value investments	135.0	0.0
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	412.6	218.5
Total mortgage and asset-backed securities and collateralized loan obligations | Level 2
Investment securities
Fair value investments	412.6	218.5
Total mortgage and asset-backed securities and collateralized loan obligations | Level 3
Investment securities
Fair value investments	$ 0.0	$ 0.0