Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net income	$ 14,400	$ 2,857	$ 57,304	$ 40,440	$ 57,304	$ 40,440	$ 19,224	$ 19,224
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization expense					28,904	20,636	16,994
Bad debt expense					86	657	199
Loss on disposal of property and equipment					9	161	55
Stock-based compensation					30,090	26,961	20,149
Amortization of premiums and accretion of discounts on investments					(1,136)	1,324	1,000
Excess tax benefits from stock-based compensation					0	0	(8,700)
Deferred income taxes					(2,521)	(2,718)	(440)
Changes in operating assets and liabilities:
Accounts receivable					(11,467)	(17,966)	(4,898)
Prepaid expenses and other assets					(4,970)	(53)	(2,107)
Accounts payable					3,515	(454)	(1,220)
Accrued liabilities					1,426	1,485	9,696
Deferred revenues					24,725	29,830	17,903
Other noncurrent liabilities					(501)	7,343	1,455
Net cash provided by operating activities					125,464	107,646	69,310
Cash flows from investing activities:
Purchases of investments					(339,862)	(299,891)	(222,953)
Sales and maturities of investments					285,224	231,996	149,708
Purchases of property and equipment					(22,775)	(37,818)	(23,245)
Payments to Acquire Businesses, Gross			(12,500)		(13,633)	(12,482)	0
Payments to Acquire Investments					2,500	0
Capitalized software development costs							0
Net cash used in investing activities					(93,546)	(118,195)	(96,490)
Payments for Repurchase of Common Stock					(85,040)	0	0
Cash flows from financing activities:
Proceeds from exercise of stock options					24,053	31,327	15,157
Payments Related to Tax Withholding for Share-based Compensation					(14,879)	(20,924)	(438)
Excess tax benefits from stock-based compensation					0	0	8,700
Repayments of Long-term Capital Lease Obligations					(1,617)	0	0
Net cash provided by financing activities					(77,483)	10,403	23,419
Cash and cash equivalents at beginning of period			86,591		86,591
Cash and cash equivalents at end of period	41,026	86,591			41,026	86,591
Supplemental disclosures of cash flow information
Cash paid for interest expense			168	27		3
Cash paid for income taxes, net of refunds			2,693	856		1,584
Non-cash investing and financing activities
Business acquisitions recorded in Intangible Assets and Accrued liabilities			4,676	0		1,000
Purchases of property and equipment recorded in accounts payable and accrued liabilities			$ 4,190	$ 1,438		2,765
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect					(45,565)	(146)	(3,761)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 42,226	$ 87,791			$ 42,226	$ 87,791	$ 87,937	$ 91,698