Settlement Assets and Obligations - Components of Investment Securities (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2020	Dec. 31, 2019
Investment Securities
Variable rate demand notes, maximum maturity year	2050
Cash and cash equivalents
Amortized Cost	$ 1,181.6	$ 1,450.5
Settlement assets:
Amortized Cost	1,839.6	1,666.8
Fair Value	1,902.0	1,698.4
Gross Unrealized Gains	62.9	32.2
Gross Unrealized Losses	(0.5)	(0.6)
Net Unrealized Gains/ (Losses)	62.4	31.6
Amortized Cost	1,862.5	1,691.4
Fair Value	1,924.9	1,723.0
Gross Unrealized Gains	62.9	32.2
Gross Unrealized Losses	(0.5)	(0.6)
Net Unrealized Gains/ (Losses)	62.4	31.6
Money market funds | Settlement Assets
Cash and cash equivalents
Amortized Cost	22.9	24.6
Fair Value	22.9	24.6
State and municipal debt securities
Settlement assets:
Amortized Cost	1,274.2	1,227.4
Fair Value	1,333.8	1,257.8
Gross Unrealized Gains	60.1	31.0
Gross Unrealized Losses	(0.5)	(0.6)
Net Unrealized Gains/ (Losses)	59.6	30.4
State and municipal variable rate demand notes
Settlement assets:
Amortized Cost	441.3	276.1
Fair Value	441.3	276.1
Corporate debt securities
Settlement assets:
Amortized Cost	65.4	52.3
Fair Value	66.5	52.4
Gross Unrealized Gains	1.1	0.1
Net Unrealized Gains/ (Losses)	1.1	0.1
United States government agency mortgage-backed securities
Settlement assets:
Amortized Cost	58.7	66.3
Fair Value	60.4	67.2
Gross Unrealized Gains	1.7	0.9
Net Unrealized Gains/ (Losses)	$ 1.7	0.9
Other United States government agency debt securities
Settlement assets:
Amortized Cost		34.9
Fair Value		34.9
United States Treasury securities
Settlement assets:
Amortized Cost		9.8
Fair Value		10.0
Gross Unrealized Gains		0.2
Net Unrealized Gains/ (Losses)		$ 0.2