Summary of Significant Accounting Policies - Customer Rewards Reserve and Revenue Recognition (Details) - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Loans and Leases Receivable Disclosure [Line Items]
Customer Reward Program	$ 3.9	$ 3.6
Minimum
Loans and Leases Receivable Disclosure [Line Items]
Partnership Agreement Initial Term	2 years
Maximum
Loans and Leases Receivable Disclosure [Line Items]
Partnership Agreement Initial Term	10 years
Credit Card Portfolio Segment [Member]
Loans and Leases Receivable Disclosure [Line Items]
Loan Commitment and Origination Fees and Discounts or Premiums, Amortization Period	12 months