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