COMMITMENTS AND CONTINGENCIES - Guarantees and Indemnifications (Details)	Jun. 30, 2019 USD ($)
Guarantee of Financial Obligations of NPT
Guarantor Obligations [Line Items]
Maximum exposure (up to)	$ 25,000,000
Guarantee of NPT Letters of Credit
Guarantor Obligations [Line Items]
Maximum exposure (up to)	$ 14,000,000

Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=118943654&loc=d3e12069-110248

Name:	us-gaap_GuaranteeObligationsMaximumExposure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant