NONCONTROLLING INTERESTS - Equity Interests (Details) - shares |
Sep. 30, 2015 |
Dec. 31, 2014 |
|---|---|---|
| Partnership Subsidiaries [Member] | ||
| Noncontrolling Interest [Line Items] | ||
| Outstanding equity interest (in shares) | 39,353,580 | 34,240,410 |
| OP units [Member] | NSA OP, LP [Member] | ||
| Noncontrolling Interest [Line Items] | ||
| Outstanding equity interest (in shares) | 21,470,876 | 18,817,088 |
| OP units [Member] | DownREIT Partnership [Member] | ||
| Noncontrolling Interest [Line Items] | ||
| Outstanding equity interest (in shares) | 1,442,466 | 1,275,979 |
| Subordinated performance units [Member] | NSA OP, LP [Member] | ||
| Noncontrolling Interest [Line Items] | ||
| Outstanding equity interest (in shares) | 9,302,989 | 8,447,679 |
| Subordinated performance units [Member] | DownREIT Partnership [Member] | ||
| Noncontrolling Interest [Line Items] | ||
| Outstanding equity interest (in shares) | 4,352,488 | 3,009,884 |
| LTIP units [Member] | NSA OP, LP [Member] | ||
| Noncontrolling Interest [Line Items] | ||
| Outstanding equity interest (in shares) | 2,784,761 | 2,689,780 |