Condensed Consolidated Statements Of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Cash flows from operating activities:
Net income	$ 400,294	$ 354,411
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation of property and equipment	36,450	37,665
Stock-based compensation	41,406	40,043
(Gain) Loss on Extinguishment of Debt	(6,554)	0
(Gain) Loss on Disposition of Business	0	(10,421)
Amortization of Debt Issuance Costs and Discounts	6,428	10,827
Accretion (Amortization) of Discounts and Premiums, Investments	(12,746)	(9,092)
Other, net	1,770	150
Changes in operating assets and liabilities
Other assets	(6,917)	4,566
Accounts payable and accrued liabilities	(29,478)	(24,756)
Deferred revenues	24,844	32,790
Deferred Income Taxes	10,662	67,385
Net cash provided by operating activities	479,267	503,568
Net Cash Provided by (Used in) Investing Activities [Abstract]
Proceeds from maturities and sales of marketable securities	3,081,702	3,895,675
Purchases of marketable securities	(2,067,498)	(4,398,787)
Purchases of property and equipment	(29,597)	(40,609)
Other investing activities	(160)	11,748
Net cash used in investing activities	984,447	(531,973)
Cash flows from financing activities:
Repayments of Subordinated Debt	(1,250,009)	0
Proceeds from employee stock purchase plans	12,836	12,915
Repurchases of common stock	(459,803)	(474,290)
Proceeds from Debt, Net of Issuance Costs	0	543,185
Net cash used in financing activities	(1,696,976)	81,810
Effect of exchange rate changes on cash and cash equivalents	(985)	1,118
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect	(234,247)	54,523
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, beginning of period	475,139	241,581
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, end of period	240,892	296,104
Supplemental cash flow disclosures:
Cash paid for interest	87,184	86,622
Cash paid for income taxes, net of refunds received	$ 84,433	$ 22,717