EXCHANGEABLE SENIOR NOTES (Details) (USD $)	3 Months Ended	9 Months Ended					0 Months Ended		0 Months Ended
	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013 Exchangeable senior notes	Sep. 30, 2012 Exchangeable senior notes	Sep. 30, 2013 2.375% Exchangeable Senior Notes due 2033	Jun. 21, 2013 2.375% Exchangeable Senior Notes due 2033 Operating Partnership	Sep. 30, 2013 2.375% Exchangeable Senior Notes due 2033 Operating Partnership	Jun. 21, 2013 2.375% Exchangeable Senior Notes due 2033 Minimum Operating Partnership	Jun. 21, 2013 2.375% Exchangeable Senior Notes due 2033 Maximum Operating Partnership	Mar. 31, 2013 3.625% Exchangeable Senior Notes due April 1, 2027 Operating Partnership
Notes payable
Principal amount of notes issued							$ 250,000,000
Interest rate (as a percent)							2.375%	2.375%
Discount rate (as a percent)							1.50%
Issuance cost							1,672,000
Amortization period							5 years
Conversion ratio, number of shares per $1,000 principal amount, numerator							17.96
Principal amount used for debt instrument conversion ratio							1,000
Redemption price as percentage of principal amount of notes plus accrued and unpaid interest							100.00%
Number of days of written notice to holders of notes required for redemption									30 days	60 days
Redemption price as percentage of principal amount of notes at request of debt holders and upon occurrence of designated event							100.00%
Information about the equity and the liability component of the convertible debt instruments
Carrying amount of equity component						14,496,000
Principal amount of liability component	250,000,000	250,000,000				250,000,000					87,663,000
Unamortized discount - equity component						(13,720,000)
Unamortized cash discount	(17,299,000)	(17,299,000)				(3,579,000)	3,750,000
Net carrying amount of liability component						232,701,000
Effective interest rate on the liability component (as a percent)						4.00%
Information relating to the contractual interest rate and amortization of the discount on the liability component
Contractual interest				1,632,000	794,000
Amortization of discount on equity component	834,000	947,000	444,000	834,000	444,000
Total interest expense recognized				$ 2,466,000	$ 1,238,000