Variable Interest Entities (Narrative) (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Variable Interest Entity [Line Items]
Maximum exposure to loss	$ 1,072.0	$ 1,410.8
Carrying Amount	414.1	566.9
Private Equity Vehicles [Member] | Energy Partners I, LP [Member]
Variable Interest Entity [Line Items]
Maximum exposure to loss	3.0	3.0
Real Estate Investment Vehicles [Member]
Variable Interest Entity [Line Items]
Equity commitments	108.0
Funded equity commitments	96.3
Maximum exposure to loss	112.2	101.8
Carrying Amount	100.4	90.3
Other Private Investment Vehicles [Member]
Variable Interest Entity [Line Items]
Equity commitments	100.6	111.4
Funded equity commitments	89.6	99.6
Carrying amount of equity investment	85.6	84.0
Maximum exposure to loss	96.7	95.8
Carrying Amount	85.6	84.0
Agency Mortgage-Backed Securitizations [Member]
Variable Interest Entity [Line Items]
Carrying Amount	1,761.9	1,002.2
Nonagency Mortgage- And Asset-Backed Securitizations [Member]
Variable Interest Entity [Line Items]
Carrying Amount	351.9	439.4
JCP Entities [Member] | Private Equity Vehicles [Member]
Variable Interest Entity [Line Items]
Equity commitments	148.1	148.1
Funded equity commitments	125.4	125.1
Carrying amount of equity investment	$ 36.3	$ 37.6