CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015
Cash flows from operating activities:
Net income	$ 18,616	$ 39,895
Adjustments to reconcile net income to net cash provided by operating activities:
Stock-based compensation expense	4,228	3,694
Depreciation	13,236	8,924
Amortization of intangibles	3,654	803
Amortization of premiums on short-term investments	1,752	455
Realized losses on short-term investments	5
Tax payments for equity award issuances	95
Deferred income taxes	(654)	(2,795)
Excess tax benefits from stock-based compensation	(1,068)
Loss on disposal of long-lived assets		4
Gain on disposal of equipment	(534)
Changes in assets and liabilities:
Accounts receivable	(13,467)	(43,021)
Prepaid expenses and other current assets	(960)	(1,499)
Income taxes receivable	9,471	6,797
Other assets	56	308
Accounts payable	(7,655)	(139)
Accrued compensation	(228)	(712)
Other liabilities	5,262	19
Deferred rent	(352)	(241)
Deferred revenue	949	(1,471)
Income taxes payable		8,715
Net cash provided by operating activities	32,406	19,736
Cash flows from investing activities:
Purchases of short-term investments	(132,157)	(611,095)
Sales and maturities of short-term investments	164,767	6,429
Purchases of property and equipment	(8,263)	(2,391)
Investment in capitalized software	(8,849)	(10,282)
Net cash provided by (used in) investing activities	15,498	(617,339)
Cash flows from financing activities:
Proceeds from issuance of common stock, net of underwriters' discount		362,082
Proceeds from issuance of treasury stock, net of underwriters' discount		282,172
Payment of offering costs		(5,182)
Repayment of credit facility borrowings	(7,500)	(10,000)
Proceeds from exercise of stock options	4,415	227
Capital lease obligations paid	(31)	(53)
Tax payments for equity award issuances	(95)
Excess tax benefits from share-based compensation	1,068	223
Net cash (used in) provided by financing activities	(2,143)	629,469
Increase in cash and cash equivalents	45,761	31,866
Cash and cash equivalents, beginning of period	114,034	162,567
Cash and cash equivalents, end of period	159,795	194,433
Supplemental cash flow disclosure:
Cash paid during the period for: Income taxes, net of refunds	3,594	16,924
Cash paid during the period for: Interest	2,380	2,189
Non-cash investing activities:
Capital lease obligations incurred		175
Accounts payable for purchases of and investment in property, equipment and capitalized software	376	1,576
Accrued compensation for investment in capitalized software	$ 226	$ 297