Consolidated Statements Of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities:
Net income	$ 814,888	$ 612,299	$ 582,489
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation of property and equipment	46,352	46,330	48,367
Stock-based compensation	48,243	50,626	52,504
Investment Income, Amortization of Discount	(6,131)	(14,777)	(18,259)
Gain (Loss) on Disposition of Business	(6,402)	(817)	(54,840)
Other Operating Activities, Cash Flow Statement	3,425	3,668	14,646
Changes in operating assets and liabilities
Prepaid expenses and other assets	(9,214)	(3,279)	1,041
Accounts payable and accrued liabilities	2,227	(24)	(2,130)
Deferred revenues	29,009	16,191	19,825
Net deferred income taxes and other long term tax liabilities	(192,214)	43,675	54,124
Net Cash Provided by (Used in) Operating Activities	730,183	753,892	697,767
Cash flows from investing activities:
Proceeds from maturities and sales of marketable securities	2,305,732	2,247,904	4,031,809
Purchases of marketable securities	(2,355,405)	(2,030,521)	(2,976,752)
Purchases of property and equipment	(43,395)	(40,316)	(37,007)
Payments for (Proceeds from) Businesses and Interest in Affiliates	20,810	(9,872)	52,240
Other investing activities	0	0	(160)
Net cash provided by (used in) investing activities	(72,258)	167,195	1,070,130
Cash flows from financing activities:
Repurchases of common stock	(777,454)	(782,583)	(638,152)
Proceeds from issuance of common stock from option exercises and employee stock purchase plans	12,577	13,152	12,836
Repayments of Subordinated Debt	0	0	(1,250,009)
Proceeds from (Payments for) Other Financing Activities	0	(872)	0
Net cash used in financing activities	(764,877)	(770,303)	(1,875,325)
Effect of exchange rate changes on cash and cash equivalents	(48)	64	(958)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect	(107,000)	150,848	(108,386)
Cash and cash equivalents at end of period	401,194	508,196
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	410,601	517,601	366,753
Supplemental cash flow disclosures:
Cash paid for interest, net of capitalized interest	87,354	87,683	117,956
Cash paid for income taxes, net of refunds received	$ 132,683	$ 89,974	$ 84,906