Condensed Consolidated Statements Of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Cash flows from operating activities:
Net income	$ 116,412	$ 107,456
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation of property and equipment	13,102	14,867
Stock-based compensation	12,563	11,759
Contingent interest payment for debenture	(7,719)	(6,544)
Amortization of Debt Issuance Costs and Discounts	3,493	3,267
Other, net	(1,436)	(1,844)
Changes in operating assets and liabilities
Other assets	14,196	2,745
Accounts payable and accrued liabilities	(59,889)	(31,537)
Deferred revenues	36,689	30,998
Deferred Income Taxes	20,775	18,477
Net cash provided by operating activities	148,186	149,644
Net Cash Provided by (Used in) Investing Activities [Abstract]
Proceeds from maturities and sales of marketable securities	1,049,795	900,810
Purchases of marketable securities	(813,459)	(874,031)
Purchases of property and equipment	(9,654)	(7,082)
Other investing activities	12,092	0
Net cash used in investing activities	238,774	19,697
Cash flows from financing activities:
Proceeds from employee stock purchase plans	7,997	8,084
Repurchases of common stock	(173,048)	(172,360)
Net cash used in financing activities	(165,051)	(164,276)
Effect of exchange rate changes on cash and cash equivalents	738	301
Net decrease in cash and cash equivalents	222,647	5,366
Cash and cash equivalents at beginning of period	231,945	228,659
Cash and cash equivalents at end of period	454,592	234,025
Supplemental cash flow disclosures:
Cash paid for interest, net of capitalized interest	28,189	27,028
Cash paid for income taxes, net of refunds received	$ 17,861	$ 13,711