Condensed Consolidated Statements Of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Cash flows from operating activities:
Net income	$ 334,116	$ 162,527
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation of property and equipment	11,232	11,593
Stock-based compensation	11,441	12,462
Accretion (Amortization) of Discounts and Premiums, Investments	(3,003)	(3,854)
Other Noncash Income (Expense)	(1,692)	147
Changes in operating assets and liabilities
Other assets	28	(226)
Accounts payable and accrued liabilities	(27,409)	(31,609)
Deferred revenues	22,807	29,219
Deferred Income Taxes	(170,844)	7,365
Net cash provided by operating activities	180,060	187,330
Net Cash Provided by (Used in) Investing Activities [Abstract]
Proceeds from maturities and sales of marketable securities	805,748	939,561
Purchases of marketable securities	(730,507)	(496,779)
Payments for (Proceeds from) Businesses and Interest in Affiliates	14,856	(2,958)
Purchases of property and equipment	(11,013)	(9,133)
Net cash used in investing activities	79,084	430,691
Cash flows from financing activities:
Proceeds from employee stock purchase plans	8,296	8,253
Repurchases of common stock	(275,623)	(204,302)
Net cash used in financing activities	(267,327)	(196,049)
Effect of Exchange Rate on Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	(1,316)	255
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect	(9,499)	422,227
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, beginning of period	517,601	366,753
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, end of period	508,102	788,980
Supplemental cash flow disclosures:
Cash paid for interest	13,151	13,063
Cash paid for income taxes, net of refunds received	$ 15,914	$ 14,185