Secured and unsecured senior debt Detail of secured and unsecured debt (Details) - USD ($) $ in Thousands	1 Months Ended	6 Months Ended
	Jul. 31, 2016	Jun. 30, 2016	Jun. 30, 2015
Summary of fixed rate/hedged and floating rate debt
Total Consolidated		$ (4,116,537)
Effective rate (as a percent)		3.52%
Future principal payments due on secured and unsecured debt
Due Current Year		$ (1,734)
Due Next Year		280,633
Due Year Three		4,708
Due Year Four		898,200
Due Year Five		507,713
Thereafter		2,458,445
Outstanding Balance		4,151,433
Unamortized (Deferred Financing Cost), (Discount)/Premium		(34,896)
Repayments of Secured Debt		233,168	$ 10,075
Secured Debt Maturing April 20, 2019 [Member]
Summary of fixed rate/hedged and floating rate debt
Total Consolidated		$ (36,386)
Effective rate (as a percent)		2.94%
Maturity Date		Apr. 20, 2019
Future principal payments due on secured and unsecured debt
Due Current Year		$ 0
Due Next Year		0
Due Year Three		0
Due Year Four		40,089
Due Year Five		0
Thereafter		0
Outstanding Balance		40,089
Unamortized (Deferred Financing Cost), (Discount)/Premium		(3,703)
Construction Loans
Future principal payments due on secured and unsecured debt
Outstanding Balance		422,083
Secured notes payable
Summary of fixed rate/hedged and floating rate debt
Total Consolidated		$ (722,794)
Stated interest rate (as a percent)		3.42%
Effective rate (as a percent)		3.46%
Future principal payments due on secured and unsecured debt
Due Current Year		$ (1,734)
Due Next Year		280,633
Due Year Three		4,708
Due Year Four		226,200
Due Year Five		107,713
Thereafter		108,445
Outstanding Balance		729,433
Unamortized (Deferred Financing Cost), (Discount)/Premium		(6,639)
Secured Debt from Bank Maturing on 20 January 2017
Summary of fixed rate/hedged and floating rate debt
Total Consolidated		$ (75,854)
Stated interest rate (as a percent)		2.44%
Effective rate (as a percent)		2.79%
Maturity Date		Jan. 20, 2017
Future principal payments due on secured and unsecured debt
Due Current Year		$ 0
Due Next Year		76,000
Due Year Three		0
Due Year Four		0
Due Year Five		0
Thereafter		0
Outstanding Balance		76,000
Unamortized (Deferred Financing Cost), (Discount)/Premium		(146)
Secured Debt Maturing on 23 August 2017 [Member]
Summary of fixed rate/hedged and floating rate debt
Total Consolidated		$ (199,627)
Effective rate (as a percent)		2.44%
Maturity Date		Aug. 23, 2017
Future principal payments due on secured and unsecured debt
Due Current Year		$ 0
Due Next Year		201,241
Due Year Three		0
Due Year Four		0
Due Year Five		0
Thereafter		0
Outstanding Balance		201,241
Unamortized (Deferred Financing Cost), (Discount)/Premium		(1,614)
Secured Debt Maturing January 28, 2019 [Member]
Summary of fixed rate/hedged and floating rate debt
Total Consolidated		$ (177,672)
Effective rate (as a percent)		1.88%
Maturity Date		Jan. 28, 2019
Future principal payments due on secured and unsecured debt
Due Current Year		$ 0
Due Next Year		0
Due Year Three		0
Due Year Four		180,753
Due Year Five		0
Thereafter		0
Outstanding Balance		180,753
Unamortized (Deferred Financing Cost), (Discount)/Premium		(3,081)
Secured Debt Other Maturing 1 April 2020
Summary of fixed rate/hedged and floating rate debt
Total Consolidated		$ (109,913)
Stated interest rate (as a percent)		7.75%
Effective rate (as a percent)		8.08%
Maturity Date		Apr. 01, 2020
Future principal payments due on secured and unsecured debt
Due Current Year		$ (864)
Due Next Year		1,832
Due Year Three		1,980
Due Year Four		2,138
Due Year Five		104,352
Thereafter		0
Outstanding Balance		111,166
Unamortized (Deferred Financing Cost), (Discount)/Premium		(1,253)
Secured Debt Maturing on January 2023
Summary of fixed rate/hedged and floating rate debt
Total Consolidated		$ (36,954)
Stated interest rate (as a percent)		4.66%
Effective rate (as a percent)		4.93%
Maturity Date		Jan. 01, 2023
Future principal payments due on secured and unsecured debt
Due Current Year		$ (861)
Due Next Year		1,540
Due Year Three		1,615
Due Year Four		1,692
Due Year Five		1,770
Thereafter		29,904
Outstanding Balance		37,382
Unamortized (Deferred Financing Cost), (Discount)/Premium		(428)
Secured Notes Payable Maturing March 10, 2023 [Member]
Summary of fixed rate/hedged and floating rate debt
Total Consolidated		$ (85,586)
Stated interest rate (as a percent)		3.93%
Effective rate (as a percent)		3.33%
Maturity Date		Mar. 10, 2023
Future principal payments due on secured and unsecured debt
Due Current Year		$ 0
Due Next Year		0
Due Year Three		1,091
Due Year Four		1,505
Due Year Five		1,566
Thereafter		77,838
Outstanding Balance		82,000
Unamortized (Deferred Financing Cost), (Discount)/Premium		3,586
Secured Notes Payable Maturing July 1, 2036 [Member]
Summary of fixed rate/hedged and floating rate debt
Total Consolidated		$ (802)
Stated interest rate (as a percent)		6.50%
Effective rate (as a percent)		6.72%
Maturity Date		Jul. 01, 2036
Future principal payments due on secured and unsecured debt
Due Current Year		$ (9)
Due Next Year		20
Due Year Three		22
Due Year Four		23
Due Year Five		25
Thereafter		703
Outstanding Balance		802
Unamortized (Deferred Financing Cost), (Discount)/Premium		0
Unsecured Debt
Summary of fixed rate/hedged and floating rate debt
Total Consolidated		$ (3,393,743)
Effective rate (as a percent)		3.54%
Future principal payments due on secured and unsecured debt
Due Current Year		$ 0
Due Next Year		0
Due Year Three		0
Due Year Four		672,000
Due Year Five		400,000
Thereafter		2,350,000
Outstanding Balance		3,422,000
Unamortized (Deferred Financing Cost), (Discount)/Premium		(28,257)
Line of Credit [Member]
Summary of fixed rate/hedged and floating rate debt
Total Consolidated		$ (72,000)
Effective rate (as a percent)		1.57%
Maturity Date		Jan. 03, 2019
Annual facility fee (as a percent)		0.20%
Future principal payments due on secured and unsecured debt
Due Current Year		$ 0
Due Next Year		0
Due Year Three		0
Due Year Four		72,000
Due Year Five		0
Thereafter		0
Outstanding Balance		72,000
Unamortized (Deferred Financing Cost), (Discount)/Premium		0
2019 Unsecured Senior Bank Term Loan
Summary of fixed rate/hedged and floating rate debt
Total Consolidated		$ (597,304)
Effective rate (as a percent)		2.20%
Maturity Date		Jan. 03, 2019
Future principal payments due on secured and unsecured debt
Due Current Year		$ 0
Due Next Year		0
Due Year Three		0
Due Year Four		600,000
Due Year Five		0
Thereafter		0
Outstanding Balance		600,000
Unamortized (Deferred Financing Cost), (Discount)/Premium		(2,696)
Unsecured Bank Term Loan 2021 [Member]
Summary of fixed rate/hedged and floating rate debt
Total Consolidated		$ (347,726)
Effective rate (as a percent)		2.22%
Maturity Date		Jan. 15, 2021
Future principal payments due on secured and unsecured debt
Due Current Year		$ 0
Due Next Year		0
Due Year Three		0
Due Year Four		0
Due Year Five		0
Thereafter		350,000
Outstanding Balance		350,000
Unamortized (Deferred Financing Cost), (Discount)/Premium		(2,274)
Unsecured Senior Notes Due in January 2020 [Member]
Summary of fixed rate/hedged and floating rate debt
Total Consolidated		$ (397,207)
Stated interest rate (as a percent)		2.75%
Effective rate (as a percent)		2.95%
Maturity Date		Jan. 15, 2020
Future principal payments due on secured and unsecured debt
Due Current Year		$ 0
Due Next Year		0
Due Year Three		0
Due Year Four		0
Due Year Five		400,000
Thereafter		0
Outstanding Balance		400,000
Unamortized (Deferred Financing Cost), (Discount)/Premium		(2,793)
4.60% unsecured senior notes payable
Summary of fixed rate/hedged and floating rate debt
Total Consolidated		$ (546,274)
Stated interest rate (as a percent)		4.60%
Effective rate (as a percent)		4.72%
Maturity Date		Apr. 01, 2022
Future principal payments due on secured and unsecured debt
Due Current Year		$ 0
Due Next Year		0
Due Year Three		0
Due Year Four		0
Due Year Five		0
Thereafter		550,000
Outstanding Balance		550,000
Unamortized (Deferred Financing Cost), (Discount)/Premium		(3,726)
3.90% unsecured senior notes payable
Summary of fixed rate/hedged and floating rate debt
Total Consolidated		$ (495,905)
Stated interest rate (as a percent)		3.90%
Effective rate (as a percent)		4.02%
Maturity Date		Jun. 15, 2023
Future principal payments due on secured and unsecured debt
Due Current Year		$ 0
Due Next Year		0
Due Year Three		0
Due Year Four		0
Due Year Five		0
Thereafter		500,000
Outstanding Balance		500,000
Unamortized (Deferred Financing Cost), (Discount)/Premium		(4,095)
4.30% Unsecured Senior Notes Payable [Member]
Summary of fixed rate/hedged and floating rate debt
Total Consolidated		$ (295,437)
Stated interest rate (as a percent)		4.30%
Effective rate (as a percent)		4.46%
Maturity Date		Jan. 15, 2026
Future principal payments due on secured and unsecured debt
Due Current Year		$ 0
Due Next Year		0
Due Year Three		0
Due Year Four		0
Due Year Five		0
Thereafter		300,000
Outstanding Balance		300,000
Unamortized (Deferred Financing Cost), (Discount)/Premium		(4,563)
3.95% Unsecured Senior Notes Payable [Member]
Summary of fixed rate/hedged and floating rate debt
Total Consolidated		$ (344,775)
Stated interest rate (as a percent)		3.95%
Effective rate (as a percent)		4.00%
Maturity Date		Jan. 15, 2027
Future principal payments due on secured and unsecured debt
Due Current Year		$ 0
Due Next Year		0
Due Year Three		0
Due Year Four		0
Due Year Five		0
Thereafter		350,000
Outstanding Balance		350,000
Unamortized (Deferred Financing Cost), (Discount)/Premium		(5,225)
Unsecured Senior Notes Due in July 2029 [Member]
Summary of fixed rate/hedged and floating rate debt
Total Consolidated		$ (297,115)
Stated interest rate (as a percent)		4.50%
Effective rate (as a percent)		4.58%
Maturity Date		Jul. 30, 2029
Future principal payments due on secured and unsecured debt
Due Current Year		$ 0
Due Next Year		0
Due Year Three		0
Due Year Four		0
Due Year Five		0
Thereafter		300,000
Outstanding Balance		300,000
Unamortized (Deferred Financing Cost), (Discount)/Premium		(2,885)
Balloon Payments
Summary of fixed rate/hedged and floating rate debt
Total Consolidated		(4,124,549)
Future principal payments due on secured and unsecured debt
Due Current Year		0
Due Next Year		277,241
Due Year Three		0
Due Year Four		892,842
Due Year Five		503,979
Thereafter		2,450,487
Outstanding Balance		4,124,549
Unamortized (Deferred Financing Cost), (Discount)/Premium		0
Principal Amortization
Summary of fixed rate/hedged and floating rate debt
Total Consolidated		(8,012)
Future principal payments due on secured and unsecured debt
Due Current Year		(1,734)
Due Next Year		3,392
Due Year Three		4,708
Due Year Four		5,358
Due Year Five		3,734
Thereafter		7,958
Outstanding Balance		26,884
Unamortized (Deferred Financing Cost), (Discount)/Premium		(34,896)
Fixed Rate/Hedged Variable-rate Debt
Summary of fixed rate/hedged and floating rate debt
Total Consolidated		(3,703,795)
Future principal payments due on secured and unsecured debt
Due Current Year		(1,734)
Due Next Year		153,392
Due Year Three		4,708
Due Year Four		605,358
Due Year Five		507,713
Thereafter		2,458,445
Outstanding Balance		3,731,350
Unamortized (Deferred Financing Cost), (Discount)/Premium		(27,555)
Unhedged Variable-rate Debt
Summary of fixed rate/hedged and floating rate debt
Total Consolidated		(412,742)
Future principal payments due on secured and unsecured debt
Due Current Year		0
Due Next Year		127,241
Due Year Three		0
Due Year Four		292,842
Due Year Five		0
Thereafter		0
Outstanding Balance		420,083
Unamortized (Deferred Financing Cost), (Discount)/Premium		$ (7,341)
London Interbank Offered Rate (LIBOR) [Member] | Line of Credit [Member]
Summary of fixed rate/hedged and floating rate debt
Debt Instrument, Description of Variable Rate Basis		LIBOR
Applicable margin (as a percent)		1.10%
London Interbank Offered Rate (LIBOR) [Member] | 2019 Unsecured Senior Bank Term Loan
Summary of fixed rate/hedged and floating rate debt
Debt Instrument, Description of Variable Rate Basis		LIBOR
Applicable margin (as a percent)		1.20%
London Interbank Offered Rate (LIBOR) [Member] | Unsecured Bank Term Loan 2021 [Member]
Summary of fixed rate/hedged and floating rate debt
Debt Instrument, Description of Variable Rate Basis		LIBOR
Applicable margin (as a percent)		1.10%
Subsequent Event | Line of Credit [Member]
Summary of fixed rate/hedged and floating rate debt
Maturity Date	Oct. 29, 2021
Subsequent Event | London Interbank Offered Rate (LIBOR) [Member] | Line of Credit [Member]
Summary of fixed rate/hedged and floating rate debt
Debt Instrument, Description of Variable Rate Basis	LIBOR
Applicable margin (as a percent)	1.00%
San Francisco Bay Area | Secured Debt from Bank Maturing on 1 July 2016 [Member] | London Interbank Offered Rate (LIBOR) [Member] | Construction Loans
Summary of fixed rate/hedged and floating rate debt
Debt Instrument, Description of Variable Rate Basis		LIBOR
Applicable margin (as a percent)		1.50%
Greater Boston market | Secured Debt Maturing on 26 January 2019 [Member] | London Interbank Offered Rate (LIBOR) [Member] | Construction Loans
Summary of fixed rate/hedged and floating rate debt
Debt Instrument, Description of Variable Rate Basis		LIBOR
Applicable margin (as a percent)		1.50%
Greater Boston market | Secured Debt Maturing January 28, 2019 [Member] | Construction Loans
Future principal payments due on secured and unsecured debt
Outstanding Balance		$ 180,753
Greater Boston market | Secured Debt Maturing January 28, 2019 [Member] | London Interbank Offered Rate (LIBOR) [Member] | Construction Loans
Summary of fixed rate/hedged and floating rate debt
Applicable margin (as a percent)		1.50%
Greater Boston market | Secured Debt Maturing on 23 August 2017 [Member] | Construction Loans
Future principal payments due on secured and unsecured debt
Outstanding Balance		$ 201,241
Greater Boston market | Secured Debt Maturing on 23 August 2017 [Member] | London Interbank Offered Rate (LIBOR) [Member] | Construction Loans
Summary of fixed rate/hedged and floating rate debt
Debt Instrument, Description of Variable Rate Basis		LIBOR
Applicable margin (as a percent)		1.35%