Secured and unsecured senior debt Summary of secured and unsecured debt (Details) - USD ($) $ in Thousands	1 Months Ended		6 Months Ended
	Jul. 31, 2016	Apr. 30, 2016	Jun. 30, 2016
Debt Instrument [Line Items]
Outstanding Balance			$ 4,151,433
Long-term Debt, Percentage Bearing Variable Interest, Amount, Net			412,742
Total Consolidated			$ 4,116,537
Percentage of Total			100.00%
Weighted-Average Interest Rate at End of Period (as a percent)			3.52%
Weighted Average Remaining Terms (in years)			5 years 9 months 18 days
Long-term Debt			$ 3,703,795
Percentage of fixed rate/hedged total debt			90.00%
Percentage of unhedged floating rate total debt			10.00%
Construction Loans
Debt Instrument [Line Items]
Outstanding Balance			$ 422,083
Secured notes payable
Debt Instrument [Line Items]
Outstanding Balance			729,433
Long-term Debt, Percentage Bearing Variable Interest, Amount, Net			340,742
Total Consolidated			$ 722,794
Percentage of Total			17.60%
Weighted-Average Interest Rate at End of Period (as a percent)			3.46%
Weighted Average Remaining Terms (in years)			2 years 9 months 18 days
Long-term Debt			$ 382,052
Stated interest rate (as a percent)			3.42%
Senior Notes [Member]
Debt Instrument [Line Items]
Long-term Debt, Percentage Bearing Variable Interest, Amount, Net			$ 0
Total Consolidated			$ 2,376,713
Percentage of Total			57.80%
Weighted-Average Interest Rate at End of Period (as a percent)			4.12%
Weighted Average Remaining Terms (in years)			7 years 8 months 12 days
Long-term Debt			$ 2,376,713
Line of Credit [Member]
Debt Instrument [Line Items]
Annual facility fee (as a percent)			0.20%
Outstanding Balance			$ 72,000
Long-term Debt, Percentage Bearing Variable Interest, Amount, Net			72,000
Total Consolidated			$ 72,000
Percentage of Total			1.70%
Weighted-Average Interest Rate at End of Period (as a percent)			1.57%
Weighted Average Remaining Terms (in years)			2 years 6 months
Long-term Debt			$ 0
2019 Unsecured Senior Bank Term Loan
Debt Instrument [Line Items]
Outstanding Balance			600,000
Long-term Debt, Percentage Bearing Variable Interest, Amount, Net			0
Total Consolidated			$ 597,304
Percentage of Total			14.50%
Weighted-Average Interest Rate at End of Period (as a percent)			2.20%
Weighted Average Remaining Terms (in years)			2 years 6 months
Long-term Debt			$ 597,304
Unsecured Bank Term Loan 2021 [Member]
Debt Instrument [Line Items]
Outstanding Balance			350,000
Long-term Debt, Percentage Bearing Variable Interest, Amount, Net			0
Total Consolidated			$ 347,726
Percentage of Total			8.40%
Weighted-Average Interest Rate at End of Period (as a percent)			2.22%
Weighted Average Remaining Terms (in years)			4 years 6 months
Long-term Debt			$ 347,726
Greater Boston market | Secured Debt Maturing on 23 August 2017 [Member] | Construction Loans
Debt Instrument [Line Items]
Outstanding Balance			201,241
Greater Boston market | Secured Debt Maturing April 20, 2019 [Member] | Construction Loans
Debt Instrument [Line Items]
Outstanding Balance			$ 40,089
London Interbank Offered Rate (LIBOR) [Member] | Line of Credit [Member]
Debt Instrument [Line Items]
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
Debt Instrument [Line Items]
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]
Debt Instrument [Line Items]
Debt Instrument, Description of Variable Rate Basis			LIBOR
Applicable margin (as a percent)			1.10%
London Interbank Offered Rate (LIBOR) [Member] | San Francisco Bay Area | Secured Debt from Bank Maturing on 1 July 2016 [Member] | Construction Loans
Debt Instrument [Line Items]
Debt Instrument, Description of Variable Rate Basis			LIBOR
Applicable margin (as a percent)			1.50%
London Interbank Offered Rate (LIBOR) [Member] | Greater Boston market | Secured Debt Maturing on 23 August 2017 [Member] | Construction Loans
Debt Instrument [Line Items]
Debt Instrument, Description of Variable Rate Basis			LIBOR
Applicable margin (as a percent)			1.35%
London Interbank Offered Rate (LIBOR) [Member] | Greater Boston market | Secured Debt Maturing on 26 January 2019 [Member] | Construction Loans
Debt Instrument [Line Items]
Debt Instrument, Description of Variable Rate Basis			LIBOR
Applicable margin (as a percent)			1.50%
London Interbank Offered Rate (LIBOR) [Member] | Greater Boston market | Secured Debt Maturing April 20, 2019 [Member] | Construction Loans
Debt Instrument [Line Items]
Debt Instrument, Description of Variable Rate Basis			LIBOR
Applicable margin (as a percent)		2.00%	2.00%
Subsequent Event | London Interbank Offered Rate (LIBOR) [Member] | Line of Credit [Member]
Debt Instrument [Line Items]
Debt Instrument, Description of Variable Rate Basis	LIBOR
Applicable margin (as a percent)	1.00%