Fair Value Measurements - Assumptions (Details) - Level 3 - Discounted Cash Flow Analysis	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Senior Unsecured Notes maturing before 2026
Fair value assumptions
Discount rate (as a percent)	4.10%	4.47%
Senior Unsecured Notes maturing 2026 and after
Fair value assumptions
Discount rate (as a percent)	4.30%	4.60%
Accrued incentives and earn-out liabilities
Fair value assumptions
Discount rate (as a percent)	6.20%	5.90%
Mortgage Loans Receivable
Fair value assumptions
Discount rate (as a percent)	8.70%	8.20%

Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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