Condensed Consolidated Statements Of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Cash flows from operating activities:
Net income	$ 657,574	$ 463,974
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation of property and equipment	34,463	34,327
Stock-based compensation	36,106	38,237
Accretion (Amortization) of Discounts and Premiums, Investments	(5,844)	(10,271)
Other Noncash Income (Expense)	2,638	(2,126)
Changes in operating assets and liabilities
Other assets	(11,107)	(12,123)
Accounts payable and accrued liabilities	(5,912)	(7,110)
Deferred revenues	27,673	24,563
Deferred Income Taxes	(195,353)	26,571
Net cash provided by operating activities	534,962	560,294
Net Cash Provided by (Used in) Investing Activities [Abstract]
Proceeds from maturities and sales of marketable securities	1,804,541	1,523,862
Purchases of marketable securities	(2,093,437)	(1,721,661)
Payments for (Proceeds from) Businesses and Interest in Affiliates	20,009	(8,530)
Purchases of property and equipment	(36,933)	(31,498)
Net cash used in investing activities	(305,820)	(237,827)
Cash flows from financing activities:
Proceeds from employee stock purchase plans	12,577	13,152
Repurchases of common stock	(603,705)	(583,485)
Net cash used in financing activities	(591,128)	(570,333)
Effect of Exchange Rate on Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	(506)	(208)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect	(362,492)	(248,074)
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	155,109	118,679
Supplemental cash flow disclosures:
Cash paid for interest	56,860	57,074
Cash paid for income taxes, net of refunds received	$ 105,258	$ 75,197