Investment securities - Mortgage-backed, Asset-backed Securities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018
Estate, Equity and Other | Level 2
Total mortgage and asset backed securities held at fair value
Total mortgage and asset-backed securities held at fair value	$ 959.9	$ 1,023.9
Mortgage-backed securities | Level 2
Total mortgage and asset backed securities held at fair value
Total mortgage and asset-backed securities held at fair value	484.5	529.4
Agency
Total mortgage and asset backed securities held at fair value
Fair value increase (decrease) due to misclassification	39.7
Agency | Level 2
Total mortgage and asset backed securities held at fair value
Total mortgage and asset-backed securities held at fair value	279.0	411.3
Federal national mortgage association | Level 2
Total mortgage and asset backed securities held at fair value
Total mortgage and asset-backed securities held at fair value	159.8	185.0
Federal home loan mortgage corporation | Level 2
Total mortgage and asset backed securities held at fair value
Total mortgage and asset-backed securities held at fair value	62.0	166.9
Government national mortgage association | Level 2
Total mortgage and asset backed securities held at fair value
Total mortgage and asset-backed securities held at fair value	57.2	59.4
Non-agency
Total mortgage and asset backed securities held at fair value
Fair value increase (decrease) due to misclassification	(39.7)
Non-agency | Level 2
Total mortgage and asset backed securities held at fair value
Total mortgage and asset-backed securities held at fair value	205.5	118.1
Residential mortgage-backed securities | Level 2
Total mortgage and asset backed securities held at fair value
Total mortgage and asset-backed securities held at fair value	57.2	32.6
Commercial mortgage-backed securities | Level 2
Total mortgage and asset backed securities held at fair value
Total mortgage and asset-backed securities held at fair value	148.3	85.5
Asset-backed securities | Level 2
Total mortgage and asset backed securities held at fair value
Total mortgage and asset-backed securities held at fair value	475.4	494.5
Collateralized loan obligations | Level 2
Total mortgage and asset backed securities held at fair value
Total mortgage and asset-backed securities held at fair value		373.0
Vehicle receivables | Level 2
Total mortgage and asset backed securities held at fair value
Total mortgage and asset-backed securities held at fair value	314.6	63.8
Credit card receivables | Level 2
Total mortgage and asset backed securities held at fair value
Total mortgage and asset-backed securities held at fair value	53.0	10.7
Other | Level 2
Total mortgage and asset backed securities held at fair value
Total mortgage and asset-backed securities held at fair value	$ 107.8	$ 47.0