CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net income (loss)	$ 30,085	$ 15,906	$ (843)
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation and amortization	88,946	93,531	86,290
Amortization of intangible assets	14,916	20,120	26,987
Amortization of debt discount and issuance costs	15,508	14,432	13,647
Stock-based compensation	43,573	45,692	60,458
Loss on disposal of property and equipment	1,141	738	1,755
Deferred income taxes	(161)	8,899	(2,149)
Tax benefit from stock-based compensation	0	290	98
Excess tax benefits from stock-based compensation	0	(2,413)	(1,813)
Restructuring	10,882	0	0
Changes in operating assets and liabilities:
Accounts receivable	(24,952)	(2,142)	(24,117)
Inventories	(743)	1,715	(450)
Prepaid expenses and other assets	5,603	(19,140)	(7,436)
Accounts payable	32,189	27,128	3,139
Accrued and other liabilities	22,537	(11,333)	9,471
Net cash provided by operating activities	239,524	193,423	165,037
Cash flows from investing activities:
Purchases of property and equipment	(36,745)	(56,264)	(55,448)
Capitalization of software and website development costs	(34,006)	(33,423)	(21,221)
Purchases of investments	(205,466)	(29,422)	(31,073)
Proceeds from the maturities of investments	45,257	28,234	62,944
Proceeds from the sales of investments	13,874	12,213	10,510
Proceeds from sale of property and equipment	21,724	14,261	1,298
Acquisition of business and intangible assets, net of cash acquired	0	0	(127)
Net cash used in investing activities	(195,362)	(64,401)	(33,117)
Cash flows from financing activities:
Proceeds from issuance of common stock upon exercise of stock options	677	2,104	3,221
Repurchases of common stock	(110,000)	(112,488)	(179,090)
Prepayment of accelerated share repurchase	0	0	(75,000)
Refund of accelerated share repurchase	0	0	38,179
Excess tax benefits from stock-based compensation	0	2,413	1,813
Principal payments of capital lease and financing obligations	(29,380)	(19,377)	(12,723)
Payment for contingent consideration liabilities	0	(1,313)	0
Proceeds from borrowings, net of issuance costs	295,211	0	0
Net cash provided by (used in) financing activities	156,508	(128,661)	(223,600)
Net increase (decrease) in cash and cash equivalents	200,670	361	(91,680)
Cash and cash equivalents, beginning of period	289,224	288,863	380,543
Cash and cash equivalents, end of period	489,894	289,224	288,863
Cash paid during the period for:
Interest	5,567	3,231	2,798
Income taxes	780	1,561	1,056
Supplemental schedule of non-cash investing / financing activities:
Net increase (decrease) in accrued purchases of property and equipment	2,693	(4,013)	3,818
Net (decrease) increase in accrued capitalized software and website development costs	(396)	(319)	892
Stock-based compensation capitalized with software and website development costs	1,373	1,560	1,247
Increase in estimated fair market value of buildings under build-to-suit leases	0	0	17,161
Property and equipment acquired under capital leases	$ 19,145	$ 23,946	$ 29,097