Overview and Summary of Significant Accounting Policies - Effects of Changes in the Presentation of Financial Statements (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating expenses:
Transaction expense	$ 4,419	$ 3,346	$ 2,610
Transaction and loan losses	1,011	1,088	809
Customer support and operations	1,364	1,267	1,110
Sales and marketing	1,128	969	937
Product development	953	834	792
General and administrative	1,155	1,028	873
Depreciation and amortization	805	724	608
Other operating expenses			48
Total operating expenses	10,967	9,256	7,787
Cash flows from investing activities:
Purchases of investments	(19,418)	(21,041)	(21,626)
Maturities and sales of investments	18,450	18,429	16,148
Funds receivable and customer accounts	$ (2,480)	$ (176)	(395)
Cash flows from financing activities:
Funds receivable and customer accounts			0
Net change			(5,873)
As Reported
Operating expenses:
Transaction expense			2,610
Transaction and loan losses			809
Customer support and operations			1,220
Sales and marketing			985
Product development			947
General and administrative			560
Depreciation and amortization			608
Other operating expenses			48
Total operating expenses			7,787
Cash flows from investing activities:
Purchases of investments			(7,542)
Maturities and sales of investments			3,318
Funds receivable and customer accounts			0
Cash flows from financing activities:
Funds receivable and customer accounts			(1,649)
Net change			(5,873)
Adjustments
Operating expenses:
Transaction expense			0
Transaction and loan losses			0
Customer support and operations			(110)
Sales and marketing			(48)
Product development			(155)
General and administrative			313
Depreciation and amortization			0
Other operating expenses			0
Total operating expenses			0
Cash flows from investing activities:
Purchases of investments			(14,084)
Maturities and sales of investments			12,830
Funds receivable and customer accounts			(395)
Cash flows from financing activities:
Funds receivable and customer accounts			1,649
Net change			$ 0