Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32687-109319

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB TOPIC 6.I)
-URI http://asc.fasb.org/extlink&oid=34349781&loc=d3e330036-122817

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 13
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32698-109319

Name:	us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Income taxes (Details) - USD ($)	9 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Sep. 30, 2017
Income taxes
Unrecognized tax benefits			$ 0
Interest and penalties accrued	$ 0
Federal income tax rate (as a percent)	34.00%
Forecast
Income taxes
Federal income tax rate (as a percent)		21.00%
Massachusetts Security Corporation
Income taxes
Corporate excise tax (as a percent)	1.32%
Corporate excise tax equal to equity apportioned (as a percent)	0.26%
Taxable income apportioned (as a percent)	8.00%