Securitization Activities - Activity Related to Securitizations Accounted for as Sales (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Securitization Activities [Abstract]
Transferred assets	$ 4,552.9	$ 5,786.0	$ 5,770.5
Proceeds on new securitizations	4,594.5	5,809.0	5,811.3
Cash flows received on retained interests	$ 28.7	$ 28.2	$ 31.2

Cash Flows between a transferor and a transferee attributable to newly transferred assets related to either a securitization, asset-backed financing arrangement, or similar transfer in which the transferor has continuing involvement with the transferred financial assets underlying the transaction (including, but not limited to, servicing, recourse, and restrictions on transferor's interests in the transferred financial assets).

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (d)(1)
-URI http://asc.fasb.org/extlink&oid=64830801&loc=d3e107207-111719

Name:	us-gaap_CashFlowsBetweenTransfereeAndTransferorProceedsFromNewTransfers
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration