BUSINESS ACQUISITIONS - Burman's Apothecary, LLC (Details) $ / shares in Units, $ in Thousands			1 Months Ended	3 Months Ended								12 Months Ended
	Jun. 19, 2015 USD ($) shares	Jun. 18, 2015 $ / shares	Jun. 30, 2014 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
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
Summary of the preliminary fair value determination of the acquired assets and liabilities
Goodwill				$ 256,318				23,148				256,318	23,148	$ 1,537
Definite-lived intangible assets
Contingent consideration liability								$ 5,409					5,409
Amortization expense												24,229	$ 4,030
Class B Nonvoting Common Stock
Business acquisition
Restricted common shares			$ 12,000
Burman's Apothecary, LLC
Business acquisition
Cash at closing	$ 77,416
Number of shares issued | shares	253,036
Restricted common shares	$ 9,578
Total	86,994
Market price (in dollars per share) | $ / shares		$ 42.06
Market price multiplier to factor in restricted nature of the shares (as a percent)		90
Purchase consideration deposited into an escrow account	$ 5,000
Deposit term into an escrow account	2 years
Acquisition-related costs charged to Selling, general, and administrative expenses												$ 860
Summary of the preliminary fair value determination of the acquired assets and liabilities
Accounts receivable	$ 17,109
Inventories	8,064
Prepaid expenses and other current assets	7,513
Property and equipment	88
Capitalized software for internal use	17,000
Definite-lived intangible assets	22,200
Accounts payable	(25,761)
Accrued expenses - compensation and benefits	(169)
Accrued expenses - other	(6)
Total identifiable net assets	46,038
Goodwill	40,956
Total acquisition price	86,994
Definite-lived intangible assets
Amount	22,200
Physician relationships | Burman's Apothecary, LLC
Summary of the preliminary fair value determination of the acquired assets and liabilities
Definite-lived intangible assets	$ 14,000
Definite-lived intangible assets
Useful Life	10 years
Amount	$ 14,000
Non-compete employment agreements | Burman's Apothecary, LLC
Summary of the preliminary fair value determination of the acquired assets and liabilities
Definite-lived intangible assets	$ 5,500
Definite-lived intangible assets
Useful Life	5 years
Amount	$ 5,500
Favorable supply agreement | Burman's Apothecary, LLC
Summary of the preliminary fair value determination of the acquired assets and liabilities
Definite-lived intangible assets	$ 2,700
Definite-lived intangible assets
Useful Life	1 year
Amount	$ 2,700