Mortgage Notes Payable, Unsecured Notes and Credit Facility (Details) - USD ($)	1 Months Ended				3 Months Ended		9 Months Ended
	Aug. 31, 2018	Jun. 30, 2018	Apr. 30, 2018	Feb. 28, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Notes Payable, Unsecured Notes and Credit Facility
Total mortgage notes payable, unsecured notes and Term Loans					$ 7,547,792,000		$ 7,547,792,000		$ 7,404,313,000
Credit Facility					56,000,000		56,000,000		0
Total mortgage notes payable, unsecured notes, Term Loans and Credit Facility					7,603,792,000		7,603,792,000		7,404,313,000
Gain (Loss) on Extinguishment of Debt					(1,678,000)	$ 0	(2,717,000)	$ (24,162,000)
Unsecured notes
Notes Payable, Unsecured Notes and Credit Facility
Fixed rate notes					5,650,000,000		5,650,000,000		5,350,000,000
Variable rate notes					300,000,000		300,000,000		300,000,000
Total mortgage notes payable, unsecured notes and Term Loans					6,200,000,000		6,200,000,000
Amount of debt discount					10,293,000		10,293,000		10,850,000
Amount of deferred financing costs, net					35,762,000		35,762,000		36,386,000
Term Loans
Notes Payable, Unsecured Notes and Credit Facility
Variable rate notes					250,000,000		250,000,000		250,000,000
Secured notes
Notes Payable, Unsecured Notes and Credit Facility
Fixed rate notes					534,629,000		534,629,000		593,987,000
Variable rate notes					813,163,000		813,163,000		910,326,000
Total mortgage notes payable, unsecured notes and Term Loans					1,191,080,000		1,191,080,000
Amount of debt discount					14,618,000		14,618,000		16,351,000
Amount of deferred financing costs, net					9,891,000		9,891,000		11,256,000
Notes Payable Maturities 2043 [Member] | Secured notes
Notes Payable, Unsecured Notes and Credit Facility
Repayments of secured mortgages				$ 15,174,000
Gain (Loss) on Extinguishment of Debt				426,000
Prepayment penalty				152,000
Write off of deferred debt issuance cost				274,000
Notes Payable Maturities 2018 | Secured notes
Notes Payable, Unsecured Notes and Credit Facility
Total mortgage notes payable, unsecured notes and Term Loans					0		0
Repayments of secured mortgages	$ 51,449,000		$ 13,380,000	$ 11,038,000
Notes Payable Maturities 2033 [Member] | Secured notes
Notes Payable, Unsecured Notes and Credit Facility
Repayments of secured mortgages		$ 15,295,000
Gain (Loss) on Extinguishment of Debt		635,000
Prepayment penalty		282,000
Write off of deferred debt issuance cost		$ 353,000
Notes Payable Maturities 2028 [Member] | Unsecured notes
Notes Payable, Unsecured Notes and Credit Facility
Total mortgage notes payable, unsecured notes and Term Loans					450,000,000		450,000,000
Notes Payable Maturities 2028 [Member] | Secured notes
Notes Payable, Unsecured Notes and Credit Facility
Repayments of secured mortgages	44,410,000
Gain (Loss) on Extinguishment of Debt	1,678,000
Notes Payable Maturities 2018 and 2028 [Member] | Secured notes
Notes Payable, Unsecured Notes and Credit Facility
Repayments of secured mortgages	$ 95,859,000
Line of Credit [Member]
Notes Payable, Unsecured Notes and Credit Facility
Credit Facility					$ 56,000,000		$ 56,000,000		$ 0