SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Other Operating Revenue (Details)	12 Months Ended
Dec. 31, 2019 item
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Number of non-utility members from whom other operating revenue is received	2

The number of non-utility members from whom other operating revenue is received.

No definition available.

Name:	tris_NumberOfNonUtilityMembersOtherOperatingRevenueReceived
Namespace Prefix:	tris_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration