Settlement Assets and Obligations - Components of Investment Securities (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Investment Securities
Variable rate demand notes, maximum maturity year	2050
Cash and cash equivalents
Amortized Cost	$ 1,003.4	$ 1,428.2
Available-for-sale securities:
Amortized Cost	1,388.7	1,918.1
Fair Value	1,437.9	1,990.6
Gross Unrealized Gains	50.1	72.5
Gross Unrealized Losses	(0.9)
Net Unrealized Gains/ (Losses)	49.2	72.5
Amortized Cost	1,396.3	1,931.2
Fair Value	1,445.5	2,003.7
Gross Unrealized Gains	50.1	72.5
Gross Unrealized Losses	(0.9)
Net Unrealized Gains/ (Losses)	49.2	72.5
Money market funds | Settlement Assets
Cash and cash equivalents
Amortized Cost	7.6	13.1
Fair Value	7.6	13.1
State and municipal debt securities
Available-for-sale securities:
Amortized Cost	1,185.0	1,234.1
Fair Value	1,232.6	1,303.9
Gross Unrealized Gains	48.3	69.8
Gross Unrealized Losses	(0.7)
Net Unrealized Gains/ (Losses)	47.6	69.8
State and municipal variable rate demand notes
Available-for-sale securities:
Amortized Cost	100.9	562.1
Fair Value	100.9	562.1
Corporate and other debt securities
Available-for-sale securities:
Amortized Cost	63.3	71.6
Fair Value	63.7	72.8
Gross Unrealized Gains	0.6	1.2
Gross Unrealized Losses	(0.2)
Net Unrealized Gains/ (Losses)	0.4	1.2
United States government agency mortgage-backed securities
Available-for-sale securities:
Amortized Cost	39.5	50.3
Fair Value	40.7	51.8
Gross Unrealized Gains	1.2	1.5
Net Unrealized Gains/ (Losses)	$ 1.2	$ 1.5