Condensed Consolidated Statements of Cash Flows - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Statement Of Cash Flows [Abstract]
Net income	$ 8.6	$ 11.2
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Deferred income tax expense (benefit)	4.1	(1.9)
Depreciation and amortization	11.8	14.4
Amortization of debt issuance costs and debt discount	0.3	0.3
Net periodic pension and postretirement cost	0.8	1.3
Non-cash share-based compensation	12.5	6.0
Other	0.8	0.5
Changes in operating assets and liabilities excluding effects of acquisitions and divestitures:
Trade and other receivables	(4.5)	(3.8)
Prepaid expenses and other current assets	(7.5)	(7.0)
Accounts payable and other accrued expenses	(2.0)	(5.8)
Deferred revenue	2.1	(0.2)
Employee compensation and benefits	(26.4)	(16.9)
Accrued interest		3.4
Accrued taxes	0.9	(8.1)
Other assets and liabilities	(0.1)	(2.2)
Net cash provided by (used in) operating activities	1.4	(8.8)
Cash Flows from Investing Activities
Purchase of customer trust funds marketable securities	(24.6)	(143.3)
Proceeds from sale and maturity of customer trust funds marketable securities	49.5	49.8
Expenditures for property, plant, and equipment	(4.9)	(4.0)
Expenditures for software and technology	(10.7)	(9.9)
Acquisition costs, net of cash acquired		(10.2)
Net cash provided by (used in) investing activities	9.3	(117.6)
Cash Flows from Financing Activities
Increase in customer trust funds obligations, net	480.8	1,916.1
Proceeds from issuance of common stock under share-based compensation plans	11.4	20.1
Repayment of long-term debt obligations	(2.7)	(1.7)
Net cash provided by financing activities	489.5	1,934.5
Effect of exchange rate changes on cash, restricted cash, and equivalents	(14.5)	3.9
Net increase in cash, restricted cash, and equivalents	485.7	1,812.0
Cash, restricted cash, and equivalents at beginning of period	1,658.6	1,106.3
Cash, restricted cash, and equivalents at end of period	2,144.3	2,918.3
Reconciliation of cash, restricted cash, and equivalents to the condensed consolidated balance sheets
Cash and equivalents	255.3	206.3
Restricted cash and equivalents included in customer trust funds	$ 1,889.0	$ 2,712.0
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	$ 2,144.3	$ 2,918.3