BUSINESS ACQUISITIONS - Valley Campus Pharmacy, Inc (Details) - USD ($) $ / shares in Units, $ in Thousands			6 Months Ended	12 Months Ended
	Jun. 01, 2016	May 31, 2016	Jun. 30, 2018	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	May 08, 2017	Dec. 31, 2014
Summary of the preliminary fair value determination of the acquired assets and liabilities
Definite-lived intangible assets							$ 7,730
Goodwill				$ 316,616	$ 609,592	$ 832,624		$ 256,318
Valley Campus Pharmacy, Inc
BUSINESS ACQUISITIONS
Cash	$ 70,117
Restricted common shares	9,507
Total	$ 79,624
Restricted common shares (in shares)	324,244
Market price (in dollars per share)		$ 32.58
Market price multiplier to factor in restricted nature of the shares (as a percent)		90.00%
Purchase consideration deposited into an escrow account	$ 3,800
Deposit term into an escrow account	12 months
Released from escrow account to the sellers			$ 3,650
Released from escrow to the company			$ 150
Acquisition-related costs charged to Selling, general, and administrative expenses				$ 410
Summary of the preliminary fair value determination of the acquired assets and liabilities
Cash	$ 2,114
Accounts receivable	16,271
Inventories	4,740
Prepaid expenses and other current assets	46
Property and equipment	200
Capitalized software for internal use	14,000
Definite-lived intangible assets	13,890
Other noncurrent assets	21
Accounts payable	(29,773)
Accrued expenses - compensation and benefits	(400)
Accrued expenses - other	(1,962)
Total identifiable net assets	19,147
Goodwill	60,477
Total acquisition price	79,624
Physician relationships | Valley Campus Pharmacy, Inc
Summary of the preliminary fair value determination of the acquired assets and liabilities
Definite-lived intangible assets	$ 7,700
Definite-lived intangible assets
Useful Life	10 years
Non-compete employment agreements | Valley Campus Pharmacy, Inc
Summary of the preliminary fair value determination of the acquired assets and liabilities
Definite-lived intangible assets	$ 4,490
Definite-lived intangible assets
Useful Life	5 years
Trade names and trademarks | Valley Campus Pharmacy, Inc
Summary of the preliminary fair value determination of the acquired assets and liabilities
Definite-lived intangible assets	$ 1,700
Definite-lived intangible assets
Useful Life	1 year