Commercial Mortgage Residual Interests (Narrative) (Details)	12 Months Ended
Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) securitization	Dec. 31, 2015 USD ($) securitization
Commercial Mortgage Residual Interests [Abstract]
Number of securitizations \| securitization		5	7
Other than temporary valuation impairment \| $	$ 0	$ 6,830,000	$ 531,000
Remaining number of securitizations \| securitization		2
Residuals at fair value \| $	$ 36,000

The charge against earnings to reduce the carrying value of an interest continued to be held by the transferor in a securitized financial instrument.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 325
-SubTopic 40
-Section 35
-Paragraph 4
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82899219&loc=d3e44297-111616

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