Summary of Significant Accounting Policies - Settlement Assets and Obligations (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Settlement assets:
Cash and cash equivalents	$ 1,264.8	$ 1,190.0
Receivables from selling agents and Business Solutions customers	1,573.9	1,327.3
Investment securities	1,350.2	1,231.8
Total settlement assets	4,188.9	3,749.1
Settlement obligations:
Money transfer, money order and payment service payables	2,789.2	2,598.2
Payables to agents	1,399.7	1,150.9
Total settlement obligations	$ 4,188.9	$ 3,749.1