SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Allowance for Uncollectible Accounts by Payor Mix and Related Aging - (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Jan. 01, 2016	Oct. 02, 2015	Jul. 03, 2015	Apr. 03, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Jan. 01, 2016	Dec. 31, 2014	Dec. 31, 2013
Allowance for uncollectible accounts disclosures
Allowance for uncollectible accounts	$ 17,514				$ 8,880				$ 17,514	$ 8,880
Sales revenue
Net service revenues	$ 145,217	$ 131,232	$ 127,366	$ 128,399	$ 124,756	$ 125,540	$ 125,193	$ 120,340	$ 532,214	$ 495,829	$ 356,912
0-90
Allowance for uncollectible accounts disclosures
Allowance for uncollectible accounts as percentage of accounts receivable									3.00%	1.00%
91-180
Allowance for uncollectible accounts disclosures
Allowance for uncollectible accounts as percentage of accounts receivable									5.00%	6.00%
181-365
Allowance for uncollectible accounts disclosures
Allowance for uncollectible accounts as percentage of accounts receivable									38.00%	23.00%
>1 yr.
Allowance for uncollectible accounts disclosures
Allowance for uncollectible accounts as percentage of accounts receivable									53.00%	74.00%
>2 yrs.
Allowance for uncollectible accounts disclosures
Allowance for uncollectible accounts as percentage of accounts receivable									100.00%	100.00%
Medicare program | 0-90
Allowance for uncollectible accounts disclosures
Allowance for uncollectible accounts as percentage of accounts receivable									3.00%	0.00%
Medicare program | 91-180
Allowance for uncollectible accounts disclosures
Allowance for uncollectible accounts as percentage of accounts receivable									6.00%	0.00%
Medicare program | 181-365
Allowance for uncollectible accounts disclosures
Allowance for uncollectible accounts as percentage of accounts receivable									36.00%	8.00%
Medicare program | >1 yr.
Allowance for uncollectible accounts disclosures
Allowance for uncollectible accounts as percentage of accounts receivable									55.00%	75.00%
Medicare program | >2 yrs.
Allowance for uncollectible accounts disclosures
Allowance for uncollectible accounts as percentage of accounts receivable									100.00%	100.00%
Medicaid and other government programs | 0-90
Allowance for uncollectible accounts disclosures
Allowance for uncollectible accounts as percentage of accounts receivable									4.00%	1.00%
Medicaid and other government programs | 91-180
Allowance for uncollectible accounts disclosures
Allowance for uncollectible accounts as percentage of accounts receivable									5.00%	8.00%
Medicaid and other government programs | 181-365
Allowance for uncollectible accounts disclosures
Allowance for uncollectible accounts as percentage of accounts receivable									44.00%	31.00%
Medicaid and other government programs | >1 yr.
Allowance for uncollectible accounts disclosures
Allowance for uncollectible accounts as percentage of accounts receivable									46.00%	66.00%
Medicaid and other government programs | >2 yrs.
Allowance for uncollectible accounts disclosures
Allowance for uncollectible accounts as percentage of accounts receivable									100.00%	100.00%
Self Pay | 0-90
Allowance for uncollectible accounts disclosures
Allowance for uncollectible accounts as percentage of accounts receivable									4.00%	1.00%
Self Pay | 91-180
Allowance for uncollectible accounts disclosures
Allowance for uncollectible accounts as percentage of accounts receivable									3.00%	12.00%
Self Pay | 181-365
Allowance for uncollectible accounts disclosures
Allowance for uncollectible accounts as percentage of accounts receivable									34.00%	51.00%
Self Pay | >1 yr.
Allowance for uncollectible accounts disclosures
Allowance for uncollectible accounts as percentage of accounts receivable									59.00%	74.00%
Self Pay | >2 yrs.
Allowance for uncollectible accounts disclosures
Allowance for uncollectible accounts as percentage of accounts receivable									100.00%	100.00%
Insurance | 0-90
Allowance for uncollectible accounts disclosures
Allowance for uncollectible accounts as percentage of accounts receivable									3.00%	5.00%
Insurance | 91-180
Allowance for uncollectible accounts disclosures
Allowance for uncollectible accounts as percentage of accounts receivable									5.00%	22.00%
Insurance | 181-365
Allowance for uncollectible accounts disclosures
Allowance for uncollectible accounts as percentage of accounts receivable									36.00%	46.00%
Insurance | >1 yr.
Allowance for uncollectible accounts disclosures
Allowance for uncollectible accounts as percentage of accounts receivable									56.00%	83.00%
Insurance | >2 yrs.
Allowance for uncollectible accounts disclosures
Allowance for uncollectible accounts as percentage of accounts receivable									100.00%	100.00%
Medicare shared savings program
Sales revenue
Net service revenues									$ 1,400	$ 1,600