Condensed Consolidated Statements of Cash Flows - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Cash flows from operating activities:
Net income	$ 137.2	$ 157.4
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	42.0	40.5
Amortization of debt issuance cost and debt discount	0.7	0.5
Realized gain on available-for-sale securities		(0.4)
Provision for accounts receivable credit losses	0.2	0.7
Provision for deferred income taxes	(5.8)	(3.8)
Stock-based compensation expense	11.7	8.3
Equity in investments	0.4	(0.5)
Impairment charge of investment	(0.6)
Changes in assets and liabilities:
Accounts receivable	(37.6)	(164.1)
Restricted cash and cash equivalents (margin deposit and clearing fund)	447.9
Income taxes receivable	53.1	45.3
Other current assets	(12.8)	0.8
Other assets	(6.1)	(6.2)
Accounts payable and accrued liabilities	(2.5)	37.4
Section 31 fees payable	(60.6)	30.1
Deferred revenue	11.0	11.4
Income taxes payable	3.1	(0.8)
Unrecognized tax benefits	9.5	6.2
Other liabilities	7.1	0.5
Net cash provided by operating activities	599.1	163.3
Cash flows from investing activities:
Acquisitions, net of cash acquired		(61.6)
Purchases of available-for-sale financial investments	(71.0)	(24.5)
Proceeds from maturities of available-for-sale financial investments	67.6	47.3
Contributions to investments		(1.6)
Purchases of property and equipment	(9.9)	(10.6)
Net cash used in investing activities	(13.3)	(51.0)
Cash flows from financing activities:
Principal payments of current portion of long term debt	(20.0)
Cash dividends on common stock	(45.3)	(40.0)
Repurchases of common stock from employee stock plans	(5.7)	(13.9)
Payment of contingent consideration from acquisition	(0.8)	(2.3)
Shares issued under employee stock purchase plan	(0.1)
Purchase of common stock	(47.6)	(119.5)
Net cash used in financing activities	(119.5)	(175.7)
Effect of foreign currency exchange rates on cash, cash equivalents, and restricted cash and cash equivalents	(0.5)	(0.7)
Increase (decrease) in cash, cash equivalents, and restricted cash and cash equivalents	465.8	(64.1)
Cash, cash equivalents, and restricted cash and cash equivalents: Beginning of period	1,057.5	229.3
Cash, cash equivalents, and restricted cash and cash equivalents: End of period	1,523.3	165.2
Supplemental disclosure of cash transactions:
Cash paid for income taxes	2.8	13.5
Cash paid for interest	$ 13.8	1.6
Supplemental disclosure of noncash investing activities:
Accounts receivable acquired		0.7
Goodwill acquired		60.4
Intangible assets acquired		20.3
Accounts payable and accrued expenses assumed		(1.4)
Deferred revenue acquired		(1.3)
Contingent consideration related to acquisitions		$ (17.5)