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Organization and Basis of Presentation - Narrative (Details) $ in Thousands | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | ---
Sep. 30, 2018 USD ($) community partnership building investment apartment project shares | Sep. 30, 2017 USD ($) | Sep. 30, 2018 USD ($) community partnership building investment apartment project shares | Sep. 30, 2017 USD ($) | Dec. 31, 2017 USD ($) investment shares | Jan. 01, 2018 USD ($) | Dec. 31, 2016 USD ($)
Real Estate Properties [Line Items] |  |  |  |  |  |  |
Buildings and improvements | $ 10,744,841 |  | $ 10,744,841 |  | $ 10,643,009 |  |
Prepaid expenses and other assets | $ 50,924 |  | $ 50,924 |  | $ 39,155 |  |
Ownership interest in partnership |  |  | 96.70% |  | 96.70% |  |
Apartment communities owned (in communities) | community | 247 |  | 247 |  |  |  |
Apartment units owned (in units) | apartment | 59,982 |  | 59,982 |  |  |  |
Ownership interest, number of commercial buildings (in commercial buildings) | building | 1 |  | 1 |  |  |  |
Ownership interest, number of active development projects (in projects) | project | 6 |  | 6 |  |  |  |
Ownership interest, number of loan investments (in investments) | investment | 3 |  | 3 |  |  |  |
Sales and maturities of marketable securities | $ 3,500 | $ 4,600 | $ 22,500 | $ 33,400 |  |  |
Marketable securities, realized gain (loss) | $ 100 | 32 | $ 700 | 1,700 |  |  |
Downreit limited partnerships consolidated by company (in partnerships) | partnership | 16 |  | 16 |  |  |  |
Communities within Downreit partnerships (in communities) | community | 8 |  | 8 |  |  |  |
Number of previously consolidated co-investments considered VIE (in investments) | investment | 8 |  | 8 |  |  |  |
Assets related to variable interest entities net of intercompany eliminations | $ 839,100 |  | $ 839,100 |  | $ 837,700 |  |
Liabilities related to variable interest entities net of intercompany eliminations | 264,100 |  | 264,100 |  | 265,500 |  |
Noncontrolling interest in variable interest entity | 64,600 |  | 64,600 |  | 66,700 |  |
Fixed rate debt carrying amount | 5,000,000 |  | 5,000,000 |  | 4,900,000 |  |
Fixed rate debt fair value | 5,000,000 |  | 5,000,000 |  | 5,000,000 |  |
Variable rate debt, carrying amount | 612,700 |  | 612,700 |  | 792,900 |  |
Variable rate debt fair value | 615,300 |  | 615,300 |  | 793,900 |  |
Investments in mortgage back securities, fair value | 130,600 |  | 130,600 |  | $ 120,700 |  |
Capitalized internal costs related to development and redevelopment projects | 4,700 | $ 4,600 | 14,200 | $ 14,000 |  |  |
Number of co-investments with financing distributions in excess of the company's investment (in investments) | investment |  |  |  |  | 1 |  |
Mortgage backed securities [Member] |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |
Investments in mortgage backed securities, carrying value | 122,523 |  | 122,523 |  | $ 109,548 |  |
Interest And Other Income [Member] |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |
Gain (loss) on sale of equity securities | 1,200 |  | 400 |  |  |  |
ASU 2016-01 [Member] |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |
Cumulative effect upon adoption |  |  |  |  | 0 |  |
ASU 2017-05 [Member] |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |
Cumulative effect upon adoption |  |  |  |  | 123,708 |  | $ 123,700
Essex Portfolio, L.P. [Member] |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |
Buildings and improvements | 10,744,841 |  | 10,744,841 |  | 10,643,009 |  |
Prepaid expenses and other assets | $ 50,924 |  | $ 50,924 |  | $ 39,155 |  |
Operating Partnership units outstanding (in units) | shares | 2,272,983 |  | 2,272,983 |  | 2,268,114 |  |
Redemption value of operating partnership units outstanding | $ 560,800 |  | $ 560,800 |  | $ 547,500 |  |
Essex Portfolio, L.P. [Member] | ASU 2016-01 [Member] |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |
Cumulative effect upon adoption |  |  |  |  | 0 |  |
Essex Portfolio, L.P. [Member] | ASU 2017-05 [Member] |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |
Cumulative effect upon adoption |  |  |  |  | 123,708 |  |
Adjustment [Member] |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |
Buildings and improvements |  |  |  |  | 13,200 |  |
Prepaid expenses and other assets | $ (10,000) |  | $ (10,000) |  | $ (9,300) |  |
Adjustment [Member] | ASU 2016-01 [Member] |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |
Cumulative effect upon adoption |  |  |  |  |  | $ 2,200 |

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