Secured and unsecured senior debt Schedule of secured construction loans (Details)	1 Months Ended	6 Months Ended
Apr. 30, 2016 Extension_Option	Jun. 30, 2016 USD ($) Extension_Option
Debt Instrument [Line Items]
Outstanding Balance		$ 4,151,433,000
Effective rate (as a percent)		3.52%
Construction Loans
Debt Instrument [Line Items]
Outstanding Balance		$ 422,083,000
Remaining Commitment		482,598,000
Total Aggregate Commitments		$ 904,681,000
Secured Debt Maturing January 28, 2019 [Member]
Debt Instrument [Line Items]
Maturity Date		Jan. 28, 2019
Outstanding Balance		$ 180,753,000
Effective rate (as a percent)		1.88%
Greater Boston market \| Construction Loans \| Secured Debt Maturing on 23 August 2017 [Member]
Debt Instrument [Line Items]
Debt Instrument, Extended Maturity Period		1 year
Outstanding Balance		$ 201,241,000
Remaining Commitment		49,159,000
Total Aggregate Commitments		$ 250,400,000
Greater Boston market \| Construction Loans \| Secured Debt Maturing January 28, 2019 [Member]
Debt Instrument [Line Items]
Debt Instrument, Extended Maturity Period		1 year
Outstanding Balance		$ 180,753,000
Remaining Commitment		169,247,000
Total Aggregate Commitments		$ 350,000,000
Debt Instrument, Number of One-Year Maturity Date Extension Option \| Extension_Option		2
Greater Boston market \| Construction Loans \| Secured Debt Maturing April 20, 2019 [Member]
Debt Instrument [Line Items]
Maturity Date		Apr. 20, 2019
Outstanding Balance		$ 40,089,000
Remaining Commitment		264,192,000
Total Aggregate Commitments		$ 304,281,000
London Interbank Offered Rate (LIBOR) [Member] \| Greater Boston market \| Construction Loans \| Secured Debt Maturing on 23 August 2017 [Member]
Debt Instrument [Line Items]
Applicable margin (as a percent)		1.35%
London Interbank Offered Rate (LIBOR) [Member] \| Greater Boston market \| Construction Loans \| Secured Debt Maturing January 28, 2019 [Member]
Debt Instrument [Line Items]
Applicable margin (as a percent)		1.50%
London Interbank Offered Rate (LIBOR) [Member] \| Greater Boston market \| Construction Loans \| Secured Debt Maturing April 20, 2019 [Member]
Debt Instrument [Line Items]
Debt Instrument, Extended Maturity Period	1 year
Applicable margin (as a percent)	2.00%	2.00%
Debt Instrument, Number of One-Year Maturity Date Extension Option \| Extension_Option	2

Debt Instrument, Extended Maturity Period

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- Definition

Debt Instrument, Maximum Borrowing Capacity

+ References

No definition available.

+ Details

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- Definition

Debt Instrument, Number of One-Year Maturity Date Extension Option

+ References

No definition available.

+ Details

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- Definition

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

+ References

No definition available.

+ Details

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- Definition

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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- Definition

Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

+ References

+ Details

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- Definition

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.

+ References

No definition available.

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- Definition

Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

+ References

+ Details

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- Definition

Amount of unused borrowing capacity under the long-term financing arrangement that is available to the entity as of the balance sheet date.

+ References

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