Variable Interest Entities (Narrative) (Details) $ in Millions	9 Months Ended
	Sep. 30, 2018 USD ($) Contract	Dec. 31, 2017 USD ($)
Variable Interest Entity [Line Items]
Automobile loan receivables securitized	$ 290.9
Carrying Amount	575.0	$ 579.6
Other private investment vehicles
Variable Interest Entity [Line Items]
Carrying amount of equity investment	160.4	133.0
Unfunded equity commitment related to investments	10.3	9.1
Carrying Amount	160.4	133.0
Agency Mortgage-Backed Securitizations
Variable Interest Entity [Line Items]
Carrying Amount	2,622.2	1,829.6
Nonagency Mortgage- And Asset-Backed Securitizations
Variable Interest Entity [Line Items]
Carrying Amount	156.6	253.2
JCP Entities | Private Equity Vehicles
Variable Interest Entity [Line Items]
Equity commitments	139.3	148.1
Funded equity commitments	121.3	126.3
Carrying amount of equity investment	$ 34.1	$ 23.7
Foursight Capital Credit Facilities | Secured Debt
Variable Interest Entity [Line Items]
Number of warehouse credit commitment | Contract	2