SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Stock-Based Compensation (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Stock options
Stock-based compensation
Vesting period	4 years
Non-cash stock compensation expense	$ 0.0	$ 0.0	$ 0.1
Restricted Stock
Stock-based compensation
Restricted shares of common stock issued (in shares)	116,404	108,278	111,474
Vesting period	4 years
Non-cash stock compensation expense	$ 5.6	$ 6.4	$ 6.1
Management \| Restricted Stock
Stock-based compensation
Non-cash stock compensation expense	$ 0.3	$ 0.5	$ 0.2

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 1
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Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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