Secured and unsecured senior debt Detail of secured and unsecured debt (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Mar. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Debt Instrument
Effective rate (as a percent)		3.76%
Future principal payments due on secured and unsecured debt
Outstanding Balance		$ 4,828,766
Unamortized (Deferred Financing Cost), (Discount)/Premium		(11,726)
Total Consolidated		4,817,040
Construction Loans
Future principal payments due on secured and unsecured debt
Outstanding Balance		$ 709,683
Secured notes payable
Debt Instrument
Stated interest rate (as a percent)		3.80%
Effective rate (as a percent)		3.80%
Future principal payments due on secured and unsecured debt
Outstanding Balance		$ 1,139,766
Unamortized (Deferred Financing Cost), (Discount)/Premium		14,124
Total Consolidated		$ 1,153,890
Secured Notes Payable Maturing on 4/1/20
Debt Instrument
Stated interest rate (as a percent)		7.75%
Effective rate (as a percent)		8.17%
Maturity Date		Apr. 01, 2020
Future principal payments due on secured and unsecured debt
Outstanding Balance		$ 108,940
Unamortized (Deferred Financing Cost), (Discount)/Premium		(835)
Total Consolidated		$ 108,105
Secured Notes Payable Maturing on 1/1/23
Debt Instrument
Stated interest rate (as a percent)		4.66%
Effective rate (as a percent)		5.03%
Maturity Date		Jan. 01, 2023
Future principal payments due on secured and unsecured debt
Outstanding Balance		$ 35,370
Unamortized (Deferred Financing Cost), (Discount)/Premium		(345)
Total Consolidated		$ 35,025
Secured Notes Payable Maturing on 3/10/23
Debt Instrument
Stated interest rate (as a percent)		3.93%
Effective rate (as a percent)		3.20%
Maturity Date		Mar. 10, 2023
Future principal payments due on secured and unsecured debt
Outstanding Balance		$ 82,000
Unamortized (Deferred Financing Cost), (Discount)/Premium		2,957
Total Consolidated		$ 84,957
Secured Notes Payable Maturing on 2/6/24
Debt Instrument
Stated interest rate (as a percent)		4.82%
Effective rate (as a percent)		3.40%
Maturity Date		Feb. 06, 2024
Future principal payments due on secured and unsecured debt
Outstanding Balance		$ 203,000
Unamortized (Deferred Financing Cost), (Discount)/Premium		16,706
Total Consolidated		$ 219,706
Secured Notes Payable Maturing on 7/1/36
Debt Instrument
Stated interest rate (as a percent)		6.50%
Effective rate (as a percent)		6.78%
Maturity Date		Jul. 01, 2036
Future principal payments due on secured and unsecured debt
Outstanding Balance		$ 773
Unamortized (Deferred Financing Cost), (Discount)/Premium		0
Total Consolidated		$ 773
Unsecured Debt
Debt Instrument
Effective rate (as a percent)		3.75%
Future principal payments due on secured and unsecured debt
Outstanding Balance		$ 3,689,000
Unamortized (Deferred Financing Cost), (Discount)/Premium		(25,850)
Total Consolidated		$ 3,663,150
2019 Unsecured Senior Bank Term Loan
Debt Instrument
Effective rate (as a percent)		2.84%
Maturity Date		Jan. 03, 2019
Future principal payments due on secured and unsecured debt
Outstanding Balance	$ 200,000	$ 200,000	$ 400,000
Unamortized (Deferred Financing Cost), (Discount)/Premium		(457)
Total Consolidated		$ 199,543
2021 Unsecured Senior Bank Term Loan
Debt Instrument
Effective rate (as a percent)		2.56%
Maturity Date		Jan. 15, 2021
Future principal payments due on secured and unsecured debt
Outstanding Balance		$ 350,000
Unamortized (Deferred Financing Cost), (Discount)/Premium		(1,683)
Total Consolidated		$ 348,317
$1.65 billion unsecured senior line of credit
Debt Instrument
Effective rate (as a percent)		2.00%
Maturity Date		Oct. 29, 2021
Future principal payments due on secured and unsecured debt
Outstanding Balance		$ 314,000
Unamortized (Deferred Financing Cost), (Discount)/Premium		0
Total Consolidated		$ 314,000
2.75% Unsecured Senior Notes Payable
Debt Instrument
Stated interest rate (as a percent)		2.75%
Effective rate (as a percent)		2.96%
Maturity Date		Jan. 15, 2020
Future principal payments due on secured and unsecured debt
Outstanding Balance		$ 400,000
Unamortized (Deferred Financing Cost), (Discount)/Premium		(1,822)
Total Consolidated		$ 398,178
4.60% Unsecured Senior Notes Payable
Debt Instrument
Stated interest rate (as a percent)		4.60%
Effective rate (as a percent)		4.75%
Maturity Date		Apr. 01, 2022
Future principal payments due on secured and unsecured debt
Outstanding Balance		$ 550,000
Unamortized (Deferred Financing Cost), (Discount)/Premium		(2,922)
Total Consolidated		$ 547,078
3.90% Unsecured Senior Notes Payable
Debt Instrument
Stated interest rate (as a percent)		3.90%
Effective rate (as a percent)		4.04%
Maturity Date		Jun. 15, 2023
Future principal payments due on secured and unsecured debt
Outstanding Balance		$ 500,000
Unamortized (Deferred Financing Cost), (Discount)/Premium		(3,381)
Total Consolidated		$ 496,619
4.30% Unsecured Senior Notes Payable
Debt Instrument
Stated interest rate (as a percent)		4.30%
Effective rate (as a percent)		4.52%
Maturity Date		Jan. 15, 2026
Future principal payments due on secured and unsecured debt
Outstanding Balance		$ 300,000
Unamortized (Deferred Financing Cost), (Discount)/Premium		(3,998)
Total Consolidated		$ 296,002
3.95% Unsecured Senior Notes Payable Due in 2027
Debt Instrument
Stated interest rate (as a percent)		3.95%
Effective rate (as a percent)		4.14%
Maturity Date		Jan. 15, 2027
Future principal payments due on secured and unsecured debt
Outstanding Balance		$ 350,000
Unamortized (Deferred Financing Cost), (Discount)/Premium		(4,638)
Total Consolidated		$ 345,362
3.95% Unsecured Senior Notes Payable Due in 2028
Debt Instrument
Stated interest rate (as a percent)	3.95%	3.95%
Effective rate (as a percent)		4.09%
Maturity Date	Jan. 15, 2028	Jan. 15, 2028
Future principal payments due on secured and unsecured debt
Outstanding Balance	$ 425,000	$ 425,000
Unamortized (Deferred Financing Cost), (Discount)/Premium		(4,334)
Total Consolidated		$ 420,666
4.50% Unsecured Senior Notes Payable
Debt Instrument
Stated interest rate (as a percent)		4.50%
Effective rate (as a percent)		4.62%
Maturity Date		Jul. 30, 2029
Future principal payments due on secured and unsecured debt
Outstanding Balance		$ 300,000
Unamortized (Deferred Financing Cost), (Discount)/Premium		(2,615)
Total Consolidated		$ 297,385
London Interbank Offered Rate (LIBOR) | 2019 Unsecured Senior Bank Term Loan
Debt Instrument
Applicable margin (as a percent)		1.20%
London Interbank Offered Rate (LIBOR) | 2021 Unsecured Senior Bank Term Loan
Debt Instrument
Applicable margin (as a percent)		1.10%
London Interbank Offered Rate (LIBOR) | $1.65 billion unsecured senior line of credit
Debt Instrument
Applicable margin (as a percent)		1.00%
Secured Notes Payable Maturing on 8/23/18 | Construction Loans
Debt Instrument
Effective rate (as a percent)		2.99%
Maturity Date		Aug. 23, 2018
Future principal payments due on secured and unsecured debt
Outstanding Balance		$ 211,940
Unamortized (Deferred Financing Cost), (Discount)/Premium		(660)
Total Consolidated		$ 211,280
Secured Notes Payable Maturing on 8/23/18 | London Interbank Offered Rate (LIBOR) | Construction Loans
Debt Instrument
Applicable margin (as a percent)		1.35%
Secured Notes Payable Maturing on 1/28/19 | Construction Loans
Debt Instrument
Effective rate (as a percent)		3.09%
Maturity Date		Jan. 28, 2019
Future principal payments due on secured and unsecured debt
Outstanding Balance		$ 317,979
Unamortized (Deferred Financing Cost), (Discount)/Premium		(1,595)
Total Consolidated		$ 316,384
Secured Notes Payable Maturing on 1/28/19 | London Interbank Offered Rate (LIBOR) | Construction Loans
Debt Instrument
Applicable margin (as a percent)		1.50%
Secured Notes Payable Maturing on 4/20/19 | Construction Loans
Debt Instrument
Effective rate (as a percent)		3.89%
Maturity Date		Apr. 20, 2019
Future principal payments due on secured and unsecured debt
Outstanding Balance		$ 179,764
Unamortized (Deferred Financing Cost), (Discount)/Premium		(2,104)
Total Consolidated		$ 177,660
Secured Notes Payable Maturing on 4/20/19 | London Interbank Offered Rate (LIBOR) | Construction Loans
Debt Instrument
Applicable margin (as a percent)		2.00%