The initial percentage points added to the reference rate to compute the variable rate on the debt instrument.

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The percentage of debt offering price at par value during the period.

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Maximum borrowing capacity under the credit facility with accordion option without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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The number of outstanding lines of credit.

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Line Of Credit Facility, Period For Extension Option

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Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

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-Name Regulation S-X (SX)
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Amount of long-term debt due after one year or the normal operating cycle, if longer, assumed at the acquisition date.

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Percentage points added to the reference rate to compute the variable rate on the debt instrument.

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No definition available.

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Amount of long-term debt before deduction of unamortized discount or premium. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt, with initial maturities beyond one year or the normal operating cycle, if longer.

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Face (par) amount of debt instrument at time of issuance.

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The average effective interest rate during the reporting period.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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No definition available.

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The amount of debt premium that was originally recognized at the issuance of the instrument that has yet to be amortized.

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Period the derivative contract is outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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Weighted average interest rate of long-term debt outstanding.

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The cash inflow from the issuance of long-term debt that is not secured by collateral. Excludes proceeds from tax exempt unsecured debt.

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Including the current and noncurrent portions, carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer). Senior note holders are paid off in full before any payments are made to junior note holders.

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The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

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Including the current and noncurrent portions, carrying value as of the balance sheet date of uncollateralized debt obligations (with maturities initially due after one year or beyond the operating cycle if longer).

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Unsecured Debt - Narrative (Details)	1 Months Ended			3 Months Ended	12 Months Ended
Unsecured Debt - Narrative (Details)	Jan. 31, 2016	Jan. 31, 2015 extension	Apr. 30, 2013 USD ($)	Sep. 30, 2012 USD ($)	Dec. 31, 2015 USD ($) line_of_credit	Dec. 31, 2014 USD ($)	Mar. 31, 2015 USD ($)	Apr. 01, 2014 USD ($)	Mar. 31, 2014 USD ($)
Debt Instrument [Line Items]
Unamortized debt issuance expense					$ 8,000,000	$ 11,900,000
Debt instrument, face amount					2,400,000,000	1,900,000,000
Unsecured debt, net					3,088,680,000	2,603,548,000
Senior notes					$ 465,000,000	465,000,000
Number of outstanding lines of credit \| line_of_credit					2
Maximum borrowing capacity					$ 1,030,000,000
Lines of credit, net					11,707,000	242,824,000
Swap [Member] \| Designated as Hedging Instrument [Member]
Debt Instrument [Line Items]
Derivative, notional amount					225,000,000
Unsecured Line of Credit [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity, accordion option					$ 1,000,000,000
Debt Instrument Basis spread on variable rate, initial					0.95%
Lines of credit, net					$ 15,000,000	229,800,000
Line of credit facility Number of one year extensions available \| extension		1
Period for extension option on line of credit		18 months
Line of Credit Working Capital [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity, accordion option					25,000,000.0
Lines of credit, net					0	16,600,000
BRE Properties, Inc. [Member]
Debt Instrument [Line Items]
Long-term debt acquired									$ 1,747,000
Unsecured Bonds 5.500% [Member]
Debt Instrument [Line Items]
Debt instrument, face amount					300,000,000	300,000,000
Unsecured Bonds 5.200% [Member]
Debt Instrument [Line Items]
Debt instrument, face amount					300,000,000	300,000,000
Unsecured Bonds 3.375% [Member]
Debt Instrument [Line Items]
Debt instrument, face amount					300,000,000	300,000,000
Unsecured Bonds 3.875% [Member]
Debt Instrument [Line Items]
Debt instrument, face amount					400,000,000	400,000,000
Unsecured Debt [Member]
Debt Instrument [Line Items]
Debt instrument, unamortized premium					14,300,000	27,500,000
Unamortized debt issuance expense					15,600,000	13,900,000
Unsecured Debt [Member] \| Senior Unsecured Notes Maturing April 1, 2025 [Member]
Debt Instrument [Line Items]
Debt instrument, face amount							$ 500,000,000.0
Interest rate, stated percentage							3.50%
Debt offering price							99.747%
Senior notes					494,800,000
Unsecured Line of Credit [Member]
Debt Instrument [Line Items]
Unamortized debt issuance expense					$ 3,300,000	$ 3,600,000
Weighted average interest rate					1.90%	1.80%
Unsecured debt, net					$ 11,707,000	$ 242,824,000
Unsecured Line of Credit [Member] \| London Interbank Offered Rate (LIBOR) [Member] \| Subsequent Event [Member]
Debt Instrument [Line Items]
Basis spread on rate (in hundredths)	0.90%
Fixed Rate Bond One [Member]
Debt Instrument [Line Items]
Debt instrument, face amount					465,000,000	465,000,000
Unsecured debt, net					$ 463,891,000	$ 463,443,000
Unsecured Bonds 4.5% [Member]
Debt Instrument [Line Items]
Effective interest rate					4.50%	4.50%
Interest rate, stated percentage					4.50%
Senior notes					$ 40,000,000	$ 40,000,000
Loans Payable [Member]
Debt Instrument [Line Items]
Debt instrument, face amount					$ 225,000,000	$ 225,000,000
Weighted average interest rate					2.40%	2.40%
Unsecured debt, net					$ 224,467,000	$ 224,130,000
Basis spread on rate (in hundredths)					1.05%
Term of loan					5 years
Loans Payable [Member] \| Variable Term Loan, Tranche One [Member]
Debt Instrument [Line Items]
Debt instrument, face amount					$ 200,000,000
Loans Payable [Member] \| Variable Term Loan, Tranche Two [Member]
Debt Instrument [Line Items]
Debt instrument, face amount					25,000,000
Senior Notes [Member] \| BRE Properties, Inc. [Member]
Debt Instrument [Line Items]
Unsecured debt, net					$ 919,100,000	931,400,000
Long-term debt acquired								$ 900,000,000
Senior Notes [Member] \| Unsecured Bonds 5.500% [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage					5.50%
Senior Notes [Member] \| Unsecured Bonds 5.200% [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage					5.20%
Senior Notes [Member] \| Unsecured Bonds 3.375% [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage					3.375%
Senior Notes [Member] \| Unsecured Bonds 3.875% [Member]
Debt Instrument [Line Items]
Debt instrument, face amount								$ 400,000,000.0
Unsecured debt, net					$ 394,500,000	393,800,000
Interest rate, stated percentage					3.875%			3.875%
Debt offering price								99.234%
Unsecured Bonds 3.25% [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage			3.25%
Debt offering price			99.152%
Proceeds from issuance of unsecured debt			$ 300,000,000.0
Long-term debt, gross					$ 295,900,000	295,500,000
Unsecured Bonds 3.625% [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage				3.625%
Debt offering price				98.99%
Proceeds from issuance of unsecured debt				$ 300,000,000.0
Long-term debt, gross					$ 296,000,000	$ 295,300,000