Overview and Summary of Significant Accounting Policies - Allowance for Transaction Losses and Negative Customer Balances (Details) - USD ($) $ in Millions |
Dec. 31, 2017 |
Dec. 31, 2016 |
---|---|---|
Valuation and Qualifying Accounts Disclosure [Line Items] | ||
Allowance for negative customer balances | $ 174 | $ 144 |
Accrued expenses and other current liabilities | ||
Valuation and Qualifying Accounts Disclosure [Line Items] | ||
Allowance for transaction losses | $ 92 | $ 78 |