ACCOUNTING POLICIES (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2013 tranche	Dec. 31, 2012
Redeemable Noncontrolling Interest [Line Items]
Fair value of operating partnership units exceeding cost	$ 129,800	$ 114,900
Number of foreign currency tranches reported at fair value	2
NHPPMBLP [Member]
Redeemable Noncontrolling Interest [Line Items]
Number of entity shares issued in exchange for redemption of OP Units	$ 0.7866
NHPPMBLP [Member] \| Class A Units
Redeemable Noncontrolling Interest [Line Items]
Limited partnership units held by third party investors (in shares)	2,255,629
Percentage of ownership interest on total units outstanding	27.00%
NHPPMBLP [Member] \| Class B Units
Redeemable Noncontrolling Interest [Line Items]
Percentage of ownership interest on total units outstanding	73.00%
Limited partnership units outstanding (in shares)	6,101,930

The number of general partner units outstanding.

The number of limited partner units outstanding.

Amount to be paid per share that is classified as temporary equity by entity upon redemption. Temporary equity is a security with redemption features that are outside the control of the issuer, is not classified as an asset or liability in conformity with GAAP, and is not mandatorily redeemable. Includes any type of security that is redeemable at a fixed or determinable price or on a fixed or determinable date or dates, is redeemable at the option of the holder, or has conditions for redemption which are not solely within the control of the issuer. If convertible, the issuer does not control the actions or events necessary to issue the maximum number of shares that could be required to be delivered under the conversion option if the holder exercises the option to convert the stock to another class of equity. If the security is a warrant or a rights issue, the warrant or rights issue is considered to be temporary equity if the issuer cannot demonstrate that it would be able to deliver upon the exercise of the option by the holder in all cases. Includes stock with put option held by ESOP and stock redeemable by holder only in the event of a change in control of the issuer.

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