EARNINGS PER SHARE - Schedule of Basic to Diluted Income Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	9 Months Ended
Sep. 28, 2019	Sep. 29, 2018	Sep. 28, 2019	Sep. 29, 2018
Reconciliation of basic to diluted loss per share
Net income attributable to Summit Inc.	$ 55,757	$ 71,289	$ 23,395	$ 53,069
Diluted net income attributable to Summit Inc.	$ 55,757	$ 71,289	$ 23,395	$ 53,069
LP Units
Reconciliation of basic to diluted loss per share
Anti dilutive shares excluded from calculation of earnings per share (in shares)	3,368,058	3,448,343	3,404,231	3,538,385
Warrants
Reconciliation of basic to diluted loss per share
Anti dilutive shares excluded from calculation of earnings per share (in shares)	0	0	100,037	0
Class A Common Stock
Reconciliation of basic to diluted loss per share
Weighted average shares of Class A stock outstanding (in shares)	112,179,137	111,641,344	112,020,275	111,288,211
Basic earnings per share (usd per share)	$ 0.50	$ 0.64	$ 0.21	$ 0.48
Weighted average shares of Class A stock outstanding (in shares)	100,037	18,631	0	34,134
Weighted average dilutive shares outstanding (in shares)	115,505,122	111,940,067	112,497,610	112,472,724
Diluted earnings per share (usd per share)	$ 0.48	$ 0.64	$ 0.21	$ 0.47
Class A Common Stock \| Stock options
Reconciliation of basic to diluted loss per share
Weighted average shares of Class A stock outstanding (in shares)	2,788,221	204,085	216,165	574,739
Class A Common Stock \| Restricted Stock Units (RSUs) [Member]
Reconciliation of basic to diluted loss per share
Weighted average shares of Class A stock outstanding (in shares)	384,571	4,515	222,533	402,950
Class A Common Stock \| Performance Shares [Member]
Reconciliation of basic to diluted loss per share
Weighted average shares of Class A stock outstanding (in shares)	53,156	71,492	38,637	172,690

Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

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The amount of net income or loss for the period per each share in instances when basic and diluted earnings per share are the same amount and reported as a single line item on the face of the financial statements. Basic earnings per share is the amount of net income or loss for the period per each share of common stock or unit outstanding during the reporting period. Diluted earnings per share includes the amount of net income or loss for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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