BUSINESS ACQUISITIONS - MedPro Rx, Inc. (Details) - USD ($) $ in Thousands		1 Months Ended	3 Months Ended								12 Months Ended
	Jun. 27, 2014	Jun. 30, 2014	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Business acquisition
Net sales			$ 986,823	$ 946,913	$ 808,011	$ 624,883	$ 612,075	$ 595,529	$ 541,675	$ 465,677	$ 3,366,631	$ 2,214,956	$ 1,515,139
Contingent consideration fair value							5,409					5,409
Summary of the preliminary fair value determination of the acquired assets and liabilities
Goodwill			256,318				23,148				256,318	23,148	$ 1,537
Contingent consideration
Contingent consideration liability							$ 5,409					5,409
Class B Nonvoting Common Stock
Business acquisition
Restricted common shares		$ 12,000
MedPro Rx, Inc.
Business acquisition
Cash at closing	$ 52,267
Number of shares issued	716,695
Restricted common shares	$ 12,000
Contingent consideration fair value	4,270		5,457								5,457
Total	68,537
Maximum payout of contingent consideration	11,500
Purchase consideration deposited into an escrow account	$ 3,500
Deposit term into an escrow account	2 years
Acquisition-related costs charged to Selling, general, and administrative expenses												$ 825
External lease term	5 years
Summary of the preliminary fair value determination of the acquired assets and liabilities
Cash and cash equivalents	$ 668
Accounts receivable	9,050
Inventories	3,819
Prepaid expenses and other current assets	204
Property and equipment	697
Capitalized software for internal use	25
Definite-lived intangible assets	37,099
Accounts payable	(3,638)
Accrued expenses - compensation and benefits	(157)
Accrued expenses - other	(865)
Total identifiable net assets	46,902
Goodwill	21,635
Total acquisition price	68,537
Definite-lived intangible assets
Amount	37,099
Contingent consideration
Contingent consideration liability	4,270		$ 5,457								$ 5,457
Contingent liability earned and paid				$ 5,750
Patient relationships | MedPro Rx, Inc.
Summary of the preliminary fair value determination of the acquired assets and liabilities
Definite-lived intangible assets	$ 24,000
Definite-lived intangible assets
Useful Life	7 years
Amount	$ 24,000
Trade names and trademarks | MedPro Rx, Inc.
Summary of the preliminary fair value determination of the acquired assets and liabilities
Definite-lived intangible assets	$ 8,700
Definite-lived intangible assets
Useful Life	10 years
Amount	$ 8,700
Non-compete employment agreements | MedPro Rx, Inc.
Summary of the preliminary fair value determination of the acquired assets and liabilities
Definite-lived intangible assets	$ 4,399
Definite-lived intangible assets
Useful Life	5 years
Amount	$ 4,399