Commitments And Contingencies Commitments and Contingencies (Details) $ in Millions	3 Months Ended
Mar. 31, 2017 USD ($)
Other Commitments [Line Items]
Capital commitment	$ 5.0
Non-Exchange Traded Equity Investment in Mortgage Originator
Other Commitments [Line Items]
Guaranty Liabilities	125.0
Flow agreement March 2015 with mortgage originator [Member]
Other Commitments [Line Items]
Purchase commitment	200.0
Purchases made in connection with purchase commitments	$ 129.4

This item represents a non-contingent liability for the fair value of an obligation to stand ready to perform over the term of a guaranty issued in the event that specified triggering events or conditions occur.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 25
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=7661617&loc=d3e11049-110243

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.15)
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.15)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 25
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=7661617&loc=d3e11019-110243

Name:	us-gaap_GuarantyLiabilities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant