SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Narrative) (Details) shares in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) Supplier shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($) shares	Jan. 01, 2018 USD ($)
Significant Accounting Policies [Line Items]
Percentage of parent and wholly-owned subsidiaries assets and liabilities included in financial statements (in Percent)	100.00%
Other-than-temporary impairments on investments	$ 0	$ 0	$ 0
Reserve for excess and obsolete inventory	1,945,000	1,260,000
Net book value of capitalized software	16,419,000	17,868,000
Reductions to remaining useful lives or write-downs of long-lived assets	0
Revenue from shipping and handling charges	$ 7,295,000	2,869,000	2,561,000
Number of suppliers providing volume purchase rebates | Supplier	2
Reduced cost of revenue, rebate earned	$ 486,000	75,000	698,000
Amount of rebate receivable recorded in balance sheet	0	0
Media expense	$ 177,387,000	130,451,000	$ 104,621,000
Period after initial mailing during which benefits are expected to be received (in Duration)	50 days
Future advertisements and promotions, costs prepaid	$ 3,426,000	2,155,000
Tenant improvement allowance	$ 1,576,000	$ 1,921,000
Common stock equivalents excluded from weighted average shares outstanding for diluted income per common share purposes (in Shares) | shares	146	335	229
Payment for income taxes	$ 28,547,000	$ 14,175,000	$ 13,508,000
Payment for interest	210,000	171,000	182,000
Non-cash capital additions	128,000	548,000	670,000
Amounts recognized in additional paid-in capital		2,936,000	2,750,000
Additional Paid-In Capital [Member]
Significant Accounting Policies [Line Items]
Amounts recognized in additional paid-in capital		2,936,000	2,750,000
Accounting Standards Update 2014-09 [Member] | Subsequent Event [Member]
Significant Accounting Policies [Line Items]
Increase in stockholder's equity				$ 1,500,000
Accounting Standards Update 2015-17 [Member]
Significant Accounting Policies [Line Items]
Adoption of new accounting standard	1,642,000
Accounting Standards Update 2016-09 [Member]
Significant Accounting Policies [Line Items]
Adoption of new accounting standard	5,491,000
Accounting Standards Update 2016-09 [Member] | Additional Paid-In Capital [Member]
Significant Accounting Policies [Line Items]
Amounts recognized in additional paid-in capital		2,936,000	$ 2,750,000
Other Accrued Expenses And Current liabilities [Member]
Significant Accounting Policies [Line Items]
Tenant improvement allowance	345,000	345,000
Non-current Liabilities [Member]
Significant Accounting Policies [Line Items]
Tenant improvement allowance	$ 1,231,000	$ 1,576,000
Inventory Purchases [Member] | Major Supplier 1 [Member]
Significant Accounting Policies [Line Items]
Percent concentration risk (in Percent)	12.00%	11.00%	17.00%
Inventory Purchases [Member] | Major Supplier 2 [Member]
Significant Accounting Policies [Line Items]
Percent concentration risk (in Percent)	8.00%	11.00%	19.00%
Fulfillment Cost [Member] | Third Party Fulfillment Provider Concentration [Member]
Significant Accounting Policies [Line Items]
Percent concentration risk (in Percent)	100.00%
Computer Software [Member] | Depreciation and Amortization [Member]
Significant Accounting Policies [Line Items]
Asset impairment charge		$ 1,297,000
Minimum [Member]
Significant Accounting Policies [Line Items]
Estimated useful life (in Duration)	2 years
Minimum [Member] | Direct Shipping Cost [Member] | Direct Shipping Provider [Member]
Significant Accounting Policies [Line Items]
Percent concentration risk (in Percent)	98.00%
Minimum [Member] | Retail Shipping Costs [Member] | Retail Shipping Provider [Member]
Significant Accounting Policies [Line Items]
Percent concentration risk (in Percent)	98.00%
Minimum [Member] | Internal-Use Software and Website Development [Member]
Significant Accounting Policies [Line Items]
Estimated useful life (in Duration)	2 years
Maximum [Member]
Significant Accounting Policies [Line Items]
Estimated useful life (in Duration)	7 years
Maximum [Member] | Internal-Use Software and Website Development [Member]
Significant Accounting Policies [Line Items]
Estimated useful life (in Duration)	5 years
Available-for-sale Securities [Member] | Government and Agency Securities and Corporate Debt Securities [Member] | Minimum [Member]
Significant Accounting Policies [Line Items]
Short term investments original maturities period	3 months
Available-for-sale Securities [Member] | Government and Agency Securities and Corporate Debt Securities [Member] | Maximum [Member]
Significant Accounting Policies [Line Items]
Short term investments original maturities period	3 years