Notes Payable (Narrative) (Details)			1 Months Ended	9 Months Ended
		Jun. 06, 2019 USD ($)	Nov. 01, 2019 USD ($)	Sep. 30, 2019 USD ($) yr	Oct. 01, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)
Notes Payable				$ 2,477,400,000		$ 2,321,600,000
Unamortized debt discounts and debt issuance costs				18,000,000.0		13,900,000
Unsecured Debt				2,432,137,000		1,836,427,000
Available amount under unsecured credit facility				$ 891,100,000
Weighted Average Interest Rate	[1]			4.00%
Weighted average maturity of indebtedness (including unsecured line of credit) (in years) | yr				5.6
Notes payable, effective interest rate		3.179%
Secured Debt				$ 45,250,000		485,176,000
Derivative, Notional Amount		$ 300,000,000.0
Debt Instrument, Interest Rate During Period				3.67%
Proceeds from Issuance of Debt		593,400,000
Debt Instrument, Face Amount		$ 600,000,000.0
Debt Instrument, Interest Rate, Stated Percentage		3.15%
Discounted notes payable face amount		99.751%
Debt Instrument, Interest Rate Before June 2026				3.84%
Debt Instrument, Interest Rate After June 2026				3.28%
Letter Of Credit [Member]
Maximum Ability to Issue Letters of Credit Under Unsecured Credit Facility				$ 50,000,000
Outstanding balance under credit facility				8,900,000
Senior Unsecured Notes [Member]
Unsecured Debt	[2]			2,332,400,000		1,736,800,000
Secured Debt [Member]
Secured Debt				45,300,000		485,200,000
Unsecured Credit Facility [Member]
Unsecured Credit Facility, Current Borrowing Capacity				600,000,000
Maximum borrowing capacity under unsecured credit facility				$ 900,000,000
Maximum term of bid rate loans (days)				P180D
Lesser of amount stated or the amount available under the unsecured credit facility				$ 450,000,000
Floating rate notes payable [Member]
Notes Payable				$ 99,700,000		99,600,000	$ 229,000,000.0
Weighted Average Interest Rate				3.10%			2.90%
Secured Notes Fixed Rate Debt due 2019 [Member] | Secured Debt [Member]
Repayments of Secured Debt				$ 439,300,000
4.78% Notes Due 2021 [Member] | Senior Unsecured Notes [Member]
Unsecured Debt				$ 249,300,000		$ 249,100,000
Notes payable, effective interest rate						4.78%
Debt Instrument, Maturity Date				Jun. 15, 2021
4.38% Conventional Mortgage Notes, due 2045 [Member] | Secured Debt [Member]
Notes payable, effective interest rate						4.38%
Secured Debt				$ 45,300,000		$ 45,900,000
Debt Instrument, Maturity Date				Sep. 01, 2045
Subsequent Event [Member]
Notes payable, effective interest rate					3.353%
Proceeds from Issuance of Debt			$ 296,600,000
Debt Instrument, Face Amount					$ 300,000,000.0
Debt Instrument, Interest Rate, Stated Percentage					3.35%
Discounted notes payable face amount			99.941%
Payment for Debt Extinguishment or Debt Prepayment Cost			$ 12,000,000
Subsequent Event [Member] | 4.78% Notes Due 2021 [Member] | Senior Unsecured Notes [Member]
Unsecured Debt					$ 250,000,000
Notes payable, effective interest rate					4.78%
Subsequent Event [Member] | 4.38% Conventional Mortgage Notes, due 2045 [Member] | Secured Debt [Member]
Notes payable, effective interest rate					4.38%
Secured Debt					$ 45,300,000

[1] Includes the effects of the applicable settled forward interest rate swaps.

[2] Unamortized debt discounts and debt issuance costs of $18.0 million and $13.9 million are included in senior unsecured and secured notes payable as of September 30, 2019 and December 31, 2018, respectively.