REVOLVING CREDIT FACILITY AND UNSECURED TERM LOANS (Details)					9 Months Ended	12 Months Ended
	Apr. 22, 2015 USD ($)	Apr. 21, 2015 USD ($)	Dec. 17, 2013 USD ($)	Jun. 20, 2011 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 09, 2011 USD ($)
SECURED CREDIT FACILITY, UNSECURED CREDIT FACILITY AND SECURED TERM LOANS
Loan procurement costs, net of amortization					$ 2,313,000	$ 2,800,000
Unsecured term loan borrowings outstanding					398,608,000	398,183,000
Term Loan Facility
SECURED CREDIT FACILITY, UNSECURED CREDIT FACILITY AND SECURED TERM LOANS
Loan procurement costs capitalized						2,300,000
Loan procurement costs, net of amortization					$ 1,400,000	1,800,000
Write off of unamortized cost						$ 100,000
Net proceeds from equity issuances added to minimum tangible net worth (as a percent)					75.00%
Term Loan Facility | Minimum
SECURED CREDIT FACILITY, UNSECURED CREDIT FACILITY AND SECURED TERM LOANS
Fixed charge coverage ratio					1.50
Tangible net worth					$ 821,211,200
Term Loan Facility | Maximum
SECURED CREDIT FACILITY, UNSECURED CREDIT FACILITY AND SECURED TERM LOANS
Total indebtedness to total asset value ratio (as a percent)					60.00%
Percentage of funds from operations that can be distributed on common shares					95.00%
Term Loan Facility | Term Loan A
SECURED CREDIT FACILITY, UNSECURED CREDIT FACILITY AND SECURED TERM LOANS
Maximum borrowing capacity				$ 100,000,000
Term of debt instrument				5 years
Term Loan Facility | Term Loan A | Baa2/BBB
SECURED CREDIT FACILITY, UNSECURED CREDIT FACILITY AND SECURED TERM LOANS
Interest rate, basis spread (as a percent)					1.30%
Variable interest rate basis					LIBOR
Term Loan Facility | Term Loan B
SECURED CREDIT FACILITY, UNSECURED CREDIT FACILITY AND SECURED TERM LOANS
Maximum borrowing capacity				$ 100,000,000
Term of debt instrument				7 years
Term Loan Facility | Term Loan B | Baa2/BBB
SECURED CREDIT FACILITY, UNSECURED CREDIT FACILITY AND SECURED TERM LOANS
Interest rate, basis spread (as a percent)					1.15%
Variable interest rate basis					LIBOR
Term Loan Facility | Unsecured term loan
SECURED CREDIT FACILITY, UNSECURED CREDIT FACILITY AND SECURED TERM LOANS
Unsecured term loan borrowings outstanding					$ 200,000,000
Credit Facility
SECURED CREDIT FACILITY, UNSECURED CREDIT FACILITY AND SECURED TERM LOANS
Effective weighted average interest rate (as a percent)					2.68%
Net proceeds from equity issuances added to minimum tangible net worth (as a percent)					75.00%
Credit Facility | Minimum
SECURED CREDIT FACILITY, UNSECURED CREDIT FACILITY AND SECURED TERM LOANS
Fixed charge coverage ratio					1.50
Tangible net worth					$ 821,211,200
Credit Facility | Maximum
SECURED CREDIT FACILITY, UNSECURED CREDIT FACILITY AND SECURED TERM LOANS
Total indebtedness to total asset value ratio (as a percent)					60.00%
Percentage of funds from operations that can be distributed on common shares					95.00%
Credit Facility | Revolver
SECURED CREDIT FACILITY, UNSECURED CREDIT FACILITY AND SECURED TERM LOANS
Maximum borrowing capacity	$ 500,000,000	$ 300,000,000					$ 300,000,000
Interest rate, basis spread (as a percent)					1.25%
Variable interest rate basis					LIBOR
Facility fee (as a percent)	0.15%	0.20%			0.15%
Remaining borrowing capacity					$ 499,300,000
Outstanding letter of credit					$ 700,000
Credit Facility | Term Loan C
SECURED CREDIT FACILITY, UNSECURED CREDIT FACILITY AND SECURED TERM LOANS
Maximum borrowing capacity							100,000,000
Repayment of outstanding debt			$ 100,000,000
Credit Facility | Term Loan D
SECURED CREDIT FACILITY, UNSECURED CREDIT FACILITY AND SECURED TERM LOANS
Maximum borrowing capacity							$ 200,000,000
Interest rate, basis spread (as a percent)					1.30%
Credit Facility | Unsecured term loan
SECURED CREDIT FACILITY, UNSECURED CREDIT FACILITY AND SECURED TERM LOANS
Unsecured term loan borrowings outstanding					$ 200,000,000