Acquisitions, Dispositions and Other Significant Transactions - Dispositions of Owned Communities and Assets Held for Sale (Details) $ in Thousands		3 Months Ended				12 Months Ended
	Dec. 20, 2018 USD ($) community	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($) community	Dec. 31, 2017 USD ($) community	Dec. 31, 2016 USD ($) community
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on sale of assets, net		$ 216,660	$ 9,833	$ 23,322	$ 43,431	$ 293,246	$ 19,273	$ 7,218
Assets held for sale		93,117				$ 93,117	$ 106,435
Sale of 3 communities
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of communities disposed of | community							3
Net cash proceeds							$ 8,200
Sales Of 50 Communities [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of communities disposed of | community								50
Net cash proceeds								$ 297,900
Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations | Sale Of 22 Communities [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of communities disposed of | community						22
Net cash proceeds						$ 380,700
Gain on sale of assets, net						$ 188,600
Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations | Sale Of 19 Communities [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of communities disposed of | community	18					19
Net cash proceeds	$ 49,000					$ 144,000
Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations | 13 Communities Held For Sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of communities classified as held for sale | community						13
Assets held for sale		93,100				$ 93,100
Mortgage notes payable
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payment for extinguishment of debt and prepayment penalties						174,000
Mortgage notes payable | Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations | 13 Communities Held For Sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Mortgage debt		$ 31,200				$ 31,200