NOTES PAYABLE (Schedule of Long-term Debt Instruments) (Details) (USD $)									6 Months Ended			6 Months Ended						6 Months Ended						6 Months Ended						6 Months Ended						6 Months Ended						0 Months Ended		6 Months Ended				0 Months Ended	6 Months Ended
	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2014 Portfolio Loan [Member] Secured Debt [Member]	Jun. 30, 2014 500 West Madison [Member]	Jun. 30, 2014 500 West Madison [Member] First Non-Revolver Tranche [Member]	Jun. 30, 2014 500 West Madison [Member] Second Non-Revolver Tranche [Member]	Jun. 30, 2014 Mortgages [Member]	Dec. 31, 2013 Mortgages [Member]	Jun. 30, 2014 Mortgages [Member] Town Center [Member]		Dec. 31, 2013 Mortgages [Member] Town Center [Member]	Jun. 30, 2014 Mortgages [Member] Town Center [Member] One-month LIBOR [Member]		Jun. 30, 2014 Mortgages [Member] Portfolio Loan [Member]		Dec. 31, 2013 Mortgages [Member] Portfolio Loan [Member]		Jun. 30, 2014 Mortgages [Member] Portfolio Loan [Member] Minimum [Member] One-month LIBOR [Member]		Jun. 30, 2014 Mortgages [Member] RBC Plaza [Member]		Dec. 31, 2013 Mortgages [Member] RBC Plaza [Member]		Jun. 30, 2014 Mortgages [Member] RBC Plaza [Member] One-month LIBOR [Member]		Jun. 30, 2014 Mortgages [Member] National Office Portfolio [Member]		Dec. 31, 2013 Mortgages [Member] National Office Portfolio [Member]		Jun. 30, 2014 Mortgages [Member] National Office Portfolio [Member] One-month LIBOR [Member]		Jun. 30, 2014 Mortgages [Member] 500 West Madison [Member]		Dec. 31, 2013 Mortgages [Member] 500 West Madison [Member]		Jun. 30, 2014 Mortgages [Member] 500 West Madison [Member] One-month LIBOR [Member]		Jun. 30, 2014 Mortgages [Member] 222 Main [Member]		Dec. 31, 2013 Mortgages [Member] 222 Main [Member]		Feb. 27, 2014 Mortgages [Member] 222 Main [Member] Secured Debt [Member]	Feb. 27, 2014 Mortgages [Member] 222 Main [Member] Secured Debt [Member] Beginning April 1, 2016 [Member]	Jun. 30, 2014 Mortgages [Member] Anchor Centre [Member]		Dec. 31, 2013 Mortgages [Member] Anchor Centre [Member]		May 22, 2014 Mortgages [Member] Anchor Centre [Member] One-month LIBOR [Member]	Jun. 30, 2014 Mortgages [Member] Anchor Centre [Member] One-month LIBOR [Member]		May 22, 2014 Mortgages [Member] Anchor Centre [Member] Secured Debt [Member]	Jun. 30, 2014 Secured Debt [Member] Portfolio Loan [Member]	Mar. 13, 2014 Secured Debt [Member] Portfolio Loan [Member]	Jun. 30, 2014 Secured Debt [Member] Portfolio Loan [Member] Revolving Credit Facility [Member]	Mar. 13, 2014 Secured Debt [Member] Portfolio Loan [Member] Revolving Credit Facility [Member]	Jun. 30, 2014 Secured Debt [Member] Portfolio Loan [Member] Revolving Credit Facility [Member] Subject to Certain Conditions [Member]	Jun. 30, 2014 Secured Debt [Member] Portfolio Loan [Member] Revolving Credit Facility [Member] Upon Meeting Certain Financial Coverage Ratio and Subject to Certain Conditions [Member]	Jun. 30, 2014 Secured Debt [Member] Portfolio Loan [Member] Non-Revolving Credit Facility [Member]	Mar. 13, 2014 Secured Debt [Member] Portfolio Loan [Member] Non-Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Principal Balance	$ 843,390,000	$ 730,690,000			$ 255,000,000		$ 843,390,000	$ 730,690,000	$ 75,000,000		$ 75,000,000			$ 130,000,000	[1]	$ 170,000,000	[1]			$ 68,730,000	[2]	$ 68,730,000	[2]			$ 161,960,000	[3]	$ 161,960,000	[3]			$ 255,000,000	[4]	$ 255,000,000	[4]			$ 102,700,000	[5]	$ 0	[5]			$ 50,000,000	[6]	$ 0	[6]
Basis Spread on Variable Rate												1.85%	[7]					1.75%	[1],[7]					1.80%	[2],[7]					1.50%	[3],[7]					1.65%	[4],[7]											1.50%	1.50%	[7]
Contractual Interest Rate																																						3.97%	[5]
Effective Interest Rate									2.87%	[7]				1.90%	[1],[7]					2.59%	[2],[7]					2.80%	[3],[7]					2.94%	[4],[7]					3.97%	[5],[7]					1.65%	[6]
Payment Type									Interest Only					Interest Only	[1]					Interest Only	[2]					Interest Only	[3]					Interest Only	[4]					(7)	[5]			interest-only	principal and interest	(8)	[6]
Maturity Date									Mar. 27, 2018	[8]				Apr. 01, 2018	[1],[8]					Feb. 01, 2017	[2],[8]					Jul. 01, 2017	[3],[8]					Dec. 16, 2018	[4],[8]					Mar. 01, 2021	[5],[8]					Jun. 01, 2017	[6],[8]
Frequency of periodic payment																																											monthly
Maximum borrowing capacity																																																				200,000,000.0		70,000,000.0				130,000,000.0
Term debt			130,000,000	215,000,000
Current borrowing capacity					20,000,000.0	20,000,000.0																																													53,200,000
Loan, amount outstanding					20,000,000															68,700,000						162,000,000																									50,000,000	130,000,000
Unused borrowing capacity, amount																				7,200,000						8,800,000																									3,200,000					37,300,000	32,700,000
Potential maximum borrowing capacity																																																					$ 350,000,000.0
Maximum borrowing capacity, percentage																																																							35.00%				65.00%
Amortization schedule of mortgage loans on real estate																																											30 years

[1] Portfolio Loan was secured by Domain Gateway, the McEwen Building, Gateway Tech Center, the Tower on Lake Carolyn and 201 Spear Street. The face amount of the Portfolio Loan is $200.0 million, of which $130.0 million is term debt and $70.0 million is revolving debt. As of June 30, 2014, the outstanding balance under the loan was $130.0 million and was composed of $130.0 million of term debt. As of June 30, 2014, an additional $37.3 million of revolving debt remained available for immediate future disbursements, subject to certain conditions set forth in the loan agreement. The remaining $32.7 million of revolving debt is available for future disbursements upon the Company meeting certain financial coverage ratios and subject to certain conditions set forth in the loan agreement. During the term of the Portfolio Loan, the Company has an option, which may be exercised up to three times, to increase the loan amount to a maximum of $350.0 million, of which 65% would be non-revolving debt and 35% would be revolving debt, with the addition of one or more properties to secure the Portfolio Loan, subject to certain conditions contained in the loan documents.

[2] As of June 30, 2014, $68.7 million had been disbursed to the Company and $7.2 million remained available for future disbursements, subject to certain conditions set forth in the loan agreement.

[3] The National Office Portfolio Mortgage Loan is secured by One Washingtonian Center, Preston Commons and Sterling Plaza. As of June 30, 2014, $162.0 million had been disbursed to the Company and $8.8 million remained available for future disbursements, subject to certain conditions set forth in the loan agreement.

[4] The 500 West Madison Mortgage Loan consists of $215.0 million of term debt, $20.0 million of non-revolving debt (the “First Non-Revolver Tranche”) and another $20.0 million of non-revolving debt (the “Second Non-Revolver Tranche”). At closing, the entire $255.0 million of the 500 West Madison Mortgage Loan was funded. The Company is obligated to pay down and have $20.0 million of availability under the First Non-Revolver Tranche by December 16, 2015 to be used for tenant improvements and leasing commissions, subject to certain terms and conditions contained in the loan documents.

[5] Monthly payments are initially interest-only. Beginning April 1, 2016, monthly payments include principal and interest with principal payments calculated using an amortization schedule of 30 years for the balance of the loan term, with the remaining principal balance, all accrued and unpaid interest and any other amounts due at maturity.

[6] See “Recent Financing Transactions – Anchor Centre Mortgage Loan.”

[7] Contractual interest rate represents the interest rate in effect under the loan as of June 30, 2014. Effective interest rate is calculated as the actual interest rate in effect as of June 30, 2014 (consisting of the contractual interest rate and the effect of interest rate swaps, if applicable), using interest rate indices as of June 30, 2014, where applicable. For further information regarding the Company's derivative instruments, see Note 8, “Derivative Instruments.”

[8] Represents the maturity date as of June 30, 2014; subject to certain conditions, the maturity dates of certain loans may be extended beyond the dates shown.