Condensed Consolidated Statements of Cash Flows - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Statement Of Cash Flows [Abstract]
Net income	$ 14.1	$ 17.5
Adjustments to reconcile net income to net cash provided by operating activities:
Deferred income tax expense (benefit)	0.3	(4.8)
Depreciation and amortization	23.9	29.0
Amortization of debt issuance costs and debt discount	0.6	0.6
Net periodic pension and postretirement cost	1.7	2.6
Non-cash share-based compensation	27.8	15.6
Other	0.9	0.8
Changes in operating assets and liabilities excluding effects of acquisitions and divestitures:
Trade and other receivables	(3.8)	(3.4)
Prepaid expenses and other current assets	(6.4)	(11.1)
Accounts payable and other accrued expenses	(1.8)	(5.7)
Deferred revenue	(1.1)	(1.3)
Employee compensation and benefits	(21.3)	(19.5)
Accrued interest	0.2	0.4
Accrued taxes	(3.7)	(8.1)
Other assets and liabilities	(7.5)	(2.4)
Net cash provided by operating activities	23.9	10.2
Cash Flows from Investing Activities
Purchase of customer trust funds marketable securities	(24.8)	(297.6)
Proceeds from sale and maturity of customer trust funds marketable securities	214.0	232.3
Expenditures for property, plant, and equipment	(9.9)	(7.7)
Expenditures for software and technology	(19.8)	(18.7)
Acquisition costs, net of cash and restricted cash acquired	(58.3)	(10.2)
Net cash provided by (used in) investing activities	101.2	(101.9)
Cash Flows from Financing Activities
(Decrease) increase in customer trust funds obligations, net	(571.4)	1,308.9
Proceeds from issuance of common stock under share-based compensation plans	51.5	44.1
Repayment of long-term debt obligations	(5.4)	(3.4)
Proceeds from revolving credit facility	295.0
Net cash (used in) provided by financing activities	(230.3)	1,349.6
Effect of exchange rate changes on cash, restricted cash, and equivalents	(12.4)	7.4
Net (decrease) increase in cash, restricted cash, and equivalents	(117.6)	1,265.3
Cash, restricted cash, and equivalents at beginning of period	1,658.6	1,106.3
Cash, restricted cash, and equivalents at end of period	1,541.0	2,371.6
Reconciliation of cash, restricted cash, and equivalents to the condensed consolidated balance sheets
Cash and equivalents	526.9	237.9
Restricted cash and equivalents included in customer trust funds	$ 1,014.1	$ 2,133.7
Restricted Cash and Cash Equivalents, Current, Asset, Statement of Financial Position [Extensible List]	us-gaap:FundsHeldForClients	us-gaap:FundsHeldForClients
Cash, restricted cash, and equivalents at end of period	$ 1,541.0	$ 2,371.6