4. Debt Unsecured Term Loan (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Unsecured term loan	$ 249,089	$ 249,036
Unsecured Term Loan [Member]
Debt Instrument [Line Items]
Unsecured term loan	$ 250,000
Debt Instrument, Maturity Date	Jan. 21, 2022
Unsecured Term Loan [Member] | Maximum [Member]
Debt Instrument [Line Items]
Unsecured Debt To Unencumbered Assets	60.00%
Unsecured Debt To Unencumbered Assets After Material Acquisition	65.00%
Ratio of Total Indebtedness To Gross Assets Value	60.00%
Ratio of Total Indebtedness To Gross Assets Value After Material Acquisition	65.00%
Unhedged Variable Rate Debt To Gross Asset Value Ratio	30.00%
Unsecured Term Loan [Member] | Minimum [Member]
Debt Instrument [Line Items]
Income from Unencumbered Assets To Unsecured Debt	12.50%
Income from Unencumbered Assets To Unsecured Debt After Material Acquisition	10.00%
Fixed Charge Coverage Ratio	1.70
Tangible Net Worth Amount	$ 2,300,000
Percentage Of Equity Offerings And Interests In Operating Partnerships To Be Added To Tangible Net Worth Threshold	75.00%