Settlement Assets and Obligations - Components of Investment Securities (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Investment Securities
Variable rate demand notes, maximum maturity year	2050
Cash and cash equivalents
Amortized Cost	$ 1,072.8	$ 1,450.5
Settlement assets:
Amortized Cost	1,615.6	1,666.8
Fair Value	1,653.8	1,698.4
Gross Unrealized Gains	40.2	32.2
Gross Unrealized Losses	(2.0)	(0.6)
Net Unrealized Gains/ (Losses)	38.2	31.6
Amortized Cost	1,639.9	1,691.4
Fair Value	1,678.1	1,723.0
Gross Unrealized Gains	40.2	32.2
Gross Unrealized Losses	(2.0)	(0.6)
Net Unrealized Gains/ (Losses)	38.2	31.6
Money market funds | Settlement Assets
Cash and cash equivalents
Amortized Cost	24.3	24.6
Fair Value	24.3	24.6
State and municipal debt securities
Settlement assets:
Amortized Cost	1,310.7	1,227.4
Fair Value	1,347.6	1,257.8
Gross Unrealized Gains	38.7	31.0
Gross Unrealized Losses	(1.8)	(0.6)
Net Unrealized Gains/ (Losses)	36.9	30.4
State and municipal variable rate demand notes
Settlement assets:
Amortized Cost	207.5	276.1
Fair Value	207.5	276.1
United States government agency mortgage-backed securities
Settlement assets:
Amortized Cost	63.1	66.3
Fair Value	64.5	67.2
Gross Unrealized Gains	1.4	0.9
Net Unrealized Gains/ (Losses)	1.4	0.9
Corporate debt securities
Settlement assets:
Amortized Cost	34.3	52.3
Fair Value	34.2	52.4
Gross Unrealized Gains	0.1	0.1
Gross Unrealized Losses	(0.2)
Net Unrealized Gains/ (Losses)	$ (0.1)	0.1
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