Real Estate Investments - Discontinued Operations and Assets Held for Sale - Income Statement (Details) $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended
	Jul. 31, 2015 property	Jun. 30, 2015 USD ($) property	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) property	Jun. 30, 2014 USD ($)	Dec. 31, 2014 property
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties held for sale | property		3		3		2
Revenues
Rental income		$ 95,450	$ 89,279	$ 190,484	$ 175,781
Other operating		1,227	1,423	2,618	2,871
Expenses
Property operating		33,927	33,635	68,189	66,466
General and administrative		6,713	5,661	13,451	11,633
Depreciation		26,552	24,491	52,940	48,158
Bad debts, net of recoveries		27	73	(181)	121
Other Income (Expense)
Interest and other income, net		147	2,035	239	2,136
Discontinued Operations
Income from discontinued operations		330	108	663	18
Impairments of real estate assets		0	(3,105)	0	(6,529)
Gain on sale of property		0	3	0	3
INCOME (LOSS) FROM DISCONTINUED OPERATIONS		$ 330	(2,994)	$ 663	(6,508)
Discontinued Operations [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties held for sale | property		2		2		2
Revenues
Rental income		$ 348	1,385	$ 690	2,969
Other operating		0	2	0	3
Revenues		348	1,387	690	2,972
Expenses
Property operating		19	793	48	1,906
General and administrative		0	6	0	13
Depreciation		0	475	0	1,030
Bad debts, net of recoveries		(1)	7	(1)	7
Total Expenses		18	1,281	47	2,956
Other Income (Expense)
Interest and other income, net		0	2	20	2
Total other income (expense)		0	2	20	2
Discontinued Operations
Income from discontinued operations		330	108	663	18
Impairments of real estate assets		0	(3,105)	0	(6,529)
Gain on sale of property		0	3	0	3
INCOME (LOSS) FROM DISCONTINUED OPERATIONS		$ 330	$ (2,994)	$ 663	$ (6,508)
Subsequent Event [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of held-for-sale properties sold | property	1