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Organization and Basis of Presentation - Narrative (Details) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2018 USD ($) community building investment partnership apartment project shares | Mar. 31, 2017 USD ($) | Dec. 31, 2017 USD ($) shares
Real Estate Properties [Line Items] |  |  |
Ownership interest in partnership (as a percent) | 96.70% |  | 96.70%
Apartment communities owned (in communities) | community | 247 |  |
Apartment units owned (in units) | apartment | 60,240 |  |
Ownership interest, number of commercial buildings (in commercial buildings) | building | 1 |  |
Ownership interest, number of active development projects (in projects) | project | 7 |  |
Sales and maturities of marketable securities | $ 9,600 | $ 24,900 |
Marketable securities, realized gain (loss) | $ 700 | 1,600 |
Downreit limited partnerships consolidated by company (in partnerships) | partnership | 16 |  |
Communities within Downreit partnerships (in communities) | community | 8 |  |
Number of previously consolidated co-investments considered VIE (in investments) | investment | 8 |  |
Assets related to variable interest entities net of intercompany eliminations | $ 845,700 |  | $ 837,700
Liabilities related to variable interest entities net of intercompany eliminations | 266,400 |  | 265,500
Noncontrolling interest in variable interest entity | 67,200 |  | 66,700
Fixed rate debt carrying amount | 5,100,000 |  | 4,900,000
Fixed rate debt fair value | 5,100,000 |  | 5,000,000
Variable rate debt, carrying amount | 612,100 |  | 792,900
Variable rate debt fair value | 615,600 |  | 793,900
Investments in mortgage back securities, fair value | 122,800 |  | 120,700
Capitalized internal costs related to development and redevelopment projects | $ 5,000 | $ 4,800 |
Number of co-investments with financing distributions in excess of the company's investment (in investments) | investment | 1 |  |
Interest And Other Income [Member] |  |  |
Real Estate Properties [Line Items] |  |  |
Gain (loss) on sale of equity securities | $ (900) |  |
Mortgage Backed Securities [Member] |  |  |
Real Estate Properties [Line Items] |  |  |
Carrying Value | $ 113,710 |  | 109,548
ASU 2017-05 [Member] |  |  |
Real Estate Properties [Line Items] |  |  |
Cumulative effect upon adoption |  |  | $ 123,708
Essex Portfolio, L.P. [Member] |  |  |
Real Estate Properties [Line Items] |  |  |
Operating Partnership units outstanding (in units) | shares | 2,273,413 |  | 2,268,114
Redemption value of operating partnership units outstanding | $ 547,200 |  | $ 547,500
Essex Portfolio, L.P. [Member] | ASU 2017-05 [Member] |  |  |
Real Estate Properties [Line Items] |  |  |
Cumulative effect upon adoption |  |  | $ 123,708

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