Condensed Consolidated Statements Of Cash Flows - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Cash flows from operating activities:
Net income	$ 157.5	$ 150.4
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation of property and equipment	11.9	11.6
Stock-based compensation	14.2	13.0
Other Noncash Income (Expense)	(0.6)	(1.1)
Changes in operating assets and liabilities
Other assets	8.3	(6.9)
Accounts payable and accrued liabilities	(14.9)	(7.6)
Deferred revenues	28.3	36.2
Deferred Income Taxes	1.2	0.5
Net cash provided by operating activities	207.1	198.3
Net Cash Provided by (Used in) Investing Activities [Abstract]
Proceeds from maturities and sales of marketable securities	696.0	793.8
Purchases of marketable securities	(164.9)	(962.3)
Purchases of property and equipment	(6.6)	(6.7)
Net cash used in investing activities	524.5	(175.2)
Cash flows from financing activities:
Proceeds from employee stock purchase plans	8.2	8.1
Repurchases of common stock	(204.6)	(185.4)
Net cash used in financing activities	(196.4)	(177.3)
Effect of Exchange Rate on Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	(0.1)	(0.2)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect	535.1	(154.4)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, beginning of period	228.8	410.6
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, end of period	763.9	256.2
Supplemental cash flow disclosures:
Cash paid for interest	13.1	13.2
Cash paid for income taxes, net of refunds received	$ 2.2	$ 17.3