Revenue - Disaggregation of revenue - (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenue
Revenues from contracts with customers	$ 4,625.1	$ 5,033.2	$ 5,382.6
Other revenues	209.9	258.9	207.3
Revenues	4,835.0	5,292.1	5,589.9
Consumer money transfers
Revenue
Revenues from contracts with customers	4,133.0	4,304.3	4,384.0
Other revenues	87.0	103.5	69.6
Revenues	4,220.0	4,407.8	4,453.6
Foreign exchange and payment services
Revenue
Revenues from contracts with customers	269.1	296.1	302.1
Other revenues	87.0	92.7	84.7
Revenues	356.1	388.8	386.8
Consumer bill payments
Revenue
Revenues from contracts with customers	156.3	357.5	621.5
Other revenues	14.0	37.3	30.8
Revenues	170.3	394.8	652.3
Other services
Revenue
Revenues from contracts with customers	66.7	75.3	75.0
Other revenues	21.9	25.4	22.2
Revenues	88.6	100.7	97.2
Other services | Speedpay | Divestitures
Revenue
Revenues		125.4	352.0
North America
Revenue
Revenues from contracts with customers	1,821.2	2,027.8	2,251.2
North America | Consumer money transfers
Revenue
Revenues from contracts with customers	1,605.5	1,653.5	1,632.3
North America | Foreign exchange and payment services
Revenue
Revenues from contracts with customers	83.6	95.4	97.6
North America | Consumer bill payments
Revenue
Revenues from contracts with customers	75.1	223.0	463.9
North America | Other services
Revenue
Revenues from contracts with customers	57.0	55.9	57.4
Europe and Russia/CIS
Revenue
Revenues from contracts with customers	1,456.5	1,484.5	1,536.5
Europe and Russia/CIS | Consumer money transfers
Revenue
Revenues from contracts with customers	1,330.0	1,350.1	1,399.5
Europe and Russia/CIS | Foreign exchange and payment services
Revenue
Revenues from contracts with customers	121.1	127.1	130.0
Europe and Russia/CIS | Consumer bill payments
Revenue
Revenues from contracts with customers	3.6	3.2	3.1
Europe and Russia/CIS | Other services
Revenue
Revenues from contracts with customers	1.8	4.1	3.9
Middle East, Africa, and South Asia
Revenue
Revenues from contracts with customers	631.9	644.2	656.2
Middle East, Africa, and South Asia | Consumer money transfers
Revenue
Revenues from contracts with customers	630.0	642.0	654.4
Middle East, Africa, and South Asia | Foreign exchange and payment services
Revenue
Revenues from contracts with customers	1.5	1.8	1.5
Middle East, Africa, and South Asia | Consumer bill payments
Revenue
Revenues from contracts with customers	0.4	0.4	0.3
Latin America and the Caribbean
Revenue
Revenues from contracts with customers	396.2	543.3	562.7
Latin America and the Caribbean | Consumer money transfers
Revenue
Revenues from contracts with customers	310.1	395.2	393.2
Latin America and the Caribbean | Foreign exchange and payment services
Revenue
Revenues from contracts with customers	2.4	3.4	3.1
Latin America and the Caribbean | Consumer bill payments
Revenue
Revenues from contracts with customers	75.8	129.4	152.7
Latin America and the Caribbean | Other services
Revenue
Revenues from contracts with customers	7.9	15.3	13.7
East Asia and Oceania
Revenue
Revenues from contracts with customers	319.3	333.4	376.0
East Asia and Oceania | Consumer money transfers
Revenue
Revenues from contracts with customers	257.4	263.5	304.6
East Asia and Oceania | Foreign exchange and payment services
Revenue
Revenues from contracts with customers	60.5	68.4	69.9
East Asia and Oceania | Consumer bill payments
Revenue
Revenues from contracts with customers	$ 1.4	$ 1.5	$ 1.5