Summary of Significant Accounting Policies - Goodwill (Details) - USD ($) |
3 Months Ended | 12 Months Ended | |
|---|---|---|---|
Dec. 31, 2018 |
Dec. 31, 2018 |
Dec. 31, 2017 |
|
| Accounting Policies [Abstract] | |||
| Goodwill impairment | $ 0 | ||
| Accumulated impairment loss | 0 | $ 0 | |
| Goodwill [Roll Forward] | |||
| Goodwill, beginning balance | 5,590,000,000 | $ 5,373,000,000 | |
| Acquired goodwill | 247,000,000 | 217,000,000 | |
| Disposed goodwill | (131,000,000) | (3,000,000) | |
| Foreign currency adjustments | (4,000,000) | 3,000,000 | |
| Goodwill, ending balance | 5,702,000,000 | 5,702,000,000 | 5,590,000,000 |
| Payments | |||
| Goodwill [Roll Forward] | |||
| Goodwill, beginning balance | 3,757,000,000 | 3,610,000,000 | |
| Acquired goodwill | 240,000,000 | 146,000,000 | |
| Disposed goodwill | 0 | 0 | |
| Foreign currency adjustments | (1,000,000) | 1,000,000 | |
| Goodwill, ending balance | 3,996,000,000 | 3,996,000,000 | 3,757,000,000 |
| Financial | |||
| Goodwill [Roll Forward] | |||
| Goodwill, beginning balance | 1,833,000,000 | 1,763,000,000 | |
| Acquired goodwill | 7,000,000 | 71,000,000 | |
| Disposed goodwill | (131,000,000) | (3,000,000) | |
| Foreign currency adjustments | (3,000,000) | 2,000,000 | |
| Goodwill, ending balance | $ 1,706,000,000 | $ 1,706,000,000 | $ 1,833,000,000 |