Loans to and Investments in Associated Companies - 54 Madison Narrative (Details) - 54 Madison	12 Months Ended
Dec. 31, 2017 Seat
Investments in and Advances to Affiliates [Line Items]
Percentage of ownership owned	48.10%
Number of seats in investment committee retained by Company	2
Number of seats in investment committee	4

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=66011266&loc=d3e33918-111571

Name:	us-gaap_EquityMethodInvestmentOwnershipPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant