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 |