ORGANIZATION AND BASIS OF PRESENTATION - Summary of General Terms and Operating Partnership's Equity Interests (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2013
Fund II
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Payments of dividends		$ 47.1
Operating Partnership | Fund I and Mervyns I
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Operating Partnership Share of Capital	22.22%
Fund Size	$ 90.0
Capital Called	86.6
Unfunded Commitment	$ 0.0
Equity Interest Held By Operating Partnership	37.78%
Preferred Return	9.00%
Capital Returned	$ 194.4
Operating Partnership | Fund II and Mervyns II
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Operating Partnership Share of Capital	20.00%
Fund Size	$ 300.0
Capital Called	300.0
Unfunded Commitment	$ 47.1
Equity Interest Held By Operating Partnership	20.00%
Preferred Return	8.00%
Capital Returned	$ 131.6
Operating Partnership | Fund III
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Operating Partnership Share of Capital	19.90%
Fund Size	$ 502.5
Capital Called	387.5
Unfunded Commitment	$ 62.5
Equity Interest Held By Operating Partnership	19.90%
Preferred Return	6.00%
Capital Returned	$ 445.7
Operating Partnership | Fund IV
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Operating Partnership Share of Capital	23.12%
Fund Size	$ 540.6
Capital Called	179.4
Unfunded Commitment	$ 361.2
Equity Interest Held By Operating Partnership	23.12%
Preferred Return	6.00%
Capital Returned	$ 101.9