Debt (Narrative) (Details) $ in Thousands							12 Months Ended
	Dec. 31, 2017 USD ($) project	Dec. 21, 2017 USD ($)	Aug. 31, 2017 USD ($)	Aug. 02, 2017 USD ($) property	Jun. 23, 2017 USD ($)	Jun. 05, 2017 USD ($)	Dec. 31, 2017 USD ($) project	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)
Debt Instrument [Line Items]
Issuance of senior notes, net of costs		$ 172,500			$ 399,500		$ 572,134	$ 241,795	$ 456,151
Number of real estate properties | project	104						104
Mortgage loans assumed with acquisition							$ 79,401	34,385	89,516
Mortgage loans refinanced							166,823	0	0
Repayments of Senior Debt							161,930	0	219,228
Maximum outstanding under revolving credit facility	$ 800,000						800,000
Mortgages Payable
Debt Instrument [Line Items]
Principal balance	491,561						491,561	466,305
Senior Notes
Debt Instrument [Line Items]
Principal balance	2,419,200						2,419,200	1,994,200
Plaza El Segundo | Mortgages Payable
Debt Instrument [Line Items]
Principal balance	$ 125,000						$ 125,000	175,000
Interest rate	3.83%						3.83%
Plaza El Segundo | Mortgages
Debt Instrument [Line Items]
Principal balance						$ 125,000					$ 175,000
Repayment of mortgage loan						$ 50,000
Interest rate						3.83%
3.25% and 4.50% Senior notes offering | Senior Notes
Debt Instrument [Line Items]
Principal balance					400,000
3.25% Senior notes | Senior Notes
Debt Instrument [Line Items]
Principal balance	$ 475,000	$ 175,000			$ 300,000		$ 475,000	0
Interest rate	3.25%	3.25%					3.25%
Note Offering Percent		99.404%			99.083%
Debt yield to maturity percent		3.323%			3.358%
4.50% notes | Senior Notes
Debt Instrument [Line Items]
Principal balance	$ 550,000				$ 100,000		$ 550,000	450,000
Interest rate	4.50%				4.50%		4.50%
Note Offering Percent					105.76%
Debt yield to maturity percent					4.143%
Primestor | Mortgages
Debt Instrument [Line Items]
Mortgage loans assumed with acquisition				$ 79,400
Fair value of mortgage loan				80,100
Sylmar Towne Center | Mortgages Payable
Debt Instrument [Line Items]
Principal balance	$ 17,362						$ 17,362	0
Interest rate	5.39%						5.39%
Sylmar Towne Center | Mortgages
Debt Instrument [Line Items]
Mortgage loans assumed with acquisition				17,500
Plaza Del Sol | Mortgages Payable
Debt Instrument [Line Items]
Principal balance	$ 8,579						$ 8,579	0
Interest rate	5.23%						5.23%
Plaza Del Sol | Mortgages
Debt Instrument [Line Items]
Mortgage loans assumed with acquisition				8,600
Azalea | Mortgages Payable
Debt Instrument [Line Items]
Principal balance	$ 40,000						$ 40,000	0
Interest rate	3.73%						3.73%
Bell Gardens | Mortgages Payable
Debt Instrument [Line Items]
Principal balance	$ 13,184						$ 13,184	0
Interest rate	4.06%						4.06%
Bell Gardens | Mortgages
Debt Instrument [Line Items]
Mortgage loans assumed with acquisition				$ 13,300
The Grove at Shrewsbury | Mortgages
Debt Instrument [Line Items]
Principal balance			$ 43,600
Interest rate			3.77%
Mortgage loans refinanced			$ 41,800
5.90% notes
Debt Instrument [Line Items]
Repayments of Senior Debt	$ 164,100
5.90% notes | Senior Notes
Debt Instrument [Line Items]
Principal balance	$ 0						$ 0	150,000		$ 150,000
Interest rate	5.90%						5.90%
Make-whole premium	$ 11,900
Accrued but unpaid interest	2,200
Revolving credit facility
Debt Instrument [Line Items]
Maximum outstanding under revolving credit facility	800,000						$ 800,000
Maximum amount of borrowings outstanding							344,000	251,500	324,000
Weighted average amount of borrowings outstanding							$ 147,500	$ 77,300	$ 109,700
Weighted average interest rate before amortization of debt fees							1.90%	1.30%	1.10%
Line of credit facility, annual facility fee	$ 1,000						$ 1,000
Primestor
Debt Instrument [Line Items]
Number of real estate properties | property				6