Variable Interest Entities (Narrative) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Variable Interest Entity [Line Items]
Automobile loan receivables securitized	$ 186.5
Maximum exposure to loss	1,854.2	$ 1,410.8
Carrying Amount	507.5	566.9
Private Equity Vehicles [Member] | Energy Partners I, LP [Member]
Variable Interest Entity [Line Items]
Maximum exposure to loss		3.0
Real Estate Investment Vehicles [Member]
Variable Interest Entity [Line Items]
Equity commitments	98.0
Funded equity commitments	86.5
Maximum exposure to loss	99.7	101.8
Carrying Amount	88.3	90.3
Other Private Investment Vehicles [Member]
Variable Interest Entity [Line Items]
Carrying amount of equity investment	132.6	84.0
Unfunded equity commitment related to investments	10.1	11.8
Maximum exposure to loss	142.7	95.8
Carrying Amount	132.6	84.0
Agency Mortgage-Backed Securitizations [Member]
Variable Interest Entity [Line Items]
Carrying Amount	2,026.7	1,002.2
Nonagency Mortgage- And Asset-Backed Securitizations [Member]
Variable Interest Entity [Line Items]
Carrying Amount	272.4	439.4
JCP Entities [Member] | Private Equity Vehicles [Member]
Variable Interest Entity [Line Items]
Equity commitments	148.1	148.1
Funded equity commitments	126.3	125.1
Carrying amount of equity investment	$ 28.1	$ 37.6