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Debt (Narrative) (Details) £ in Millions |  |  | 1 Months Ended | 12 Months Ended |  |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Oct. 31, 2018 USD ($) | Sep. 18, 2018 USD ($) | Jan. 31, 2019 GBP (£) | Oct. 31, 2018 | Apr. 30, 2017 USD ($) | Dec. 31, 2018 USD ($) | Jan. 31, 2019 USD ($) | Oct. 01, 2018 USD ($) | Sep. 30, 2018 USD ($) | Sep. 18, 2018 GBP (£) | Mar. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Jan. 31, 2017 USD ($)
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Commercial paper outstanding |  |  |  |  |  | $ 0 |  |  |  |  |  |  |
Debt maturity |  |  |  |  | $ 250,000,000 |  |  |  |  |  |  |  |
3.75% Senior Debt Obligations Due 2026 |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate |  |  |  |  |  | 3.75% |  |  |  |  |  |  |
4.20% Senior Debt Obligations Due 2048 |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument, amount |  |  |  |  |  |  |  |  |  |  | $ 600,000,000 |  |
Interest rate |  |  |  |  |  | 4.20% |  |  |  |  | 4.20% |  |
Amended Revolving Credit Facility November 24, 2015 |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument, term |  |  |  | 5 years |  |  |  |  |  |  |  |  |
Revolving credit facility, borrowing capacity |  |  |  |  |  |  |  |  | $ 1,500,000,000.0 |  |  |  |
Revolving credit facility, amount outstanding |  |  |  |  |  | $ 0 |  |  |  |  |  |  |
Amended Revolving Credit Facility March 28, 2014 |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revolving credit facility, borrowing capacity |  |  |  |  |  |  |  | $ 1,800,000,000.0 |  |  |  |  |
Other Debt Facilities |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revolving credit facility, amount outstanding |  |  |  |  |  | 0 |  |  |  |  |  | $ 0 |
Debt instrument, unused borrowing capacity |  |  |  |  |  | 594,000,000 |  |  |  |  |  | $ 624,000,000 |
Commercial Paper |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Short-term debt |  |  |  |  |  | $ 1,500,000,000 |  |  |  |  |  |  |
Bridge Loan |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Duration fee ninety days after first borrowing, percent |  |  |  |  |  | 50.00% |  |  |  |  |  |  |
Duration fee one hundred and eighty days after first borrowing, percent |  |  |  |  |  | 75.00% |  |  |  |  |  |  |
Duration fee two hundred and seventy days after first borrowing, percent |  |  |  |  |  | 100.00% |  |  |  |  |  |  |
Senior Notes |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt maturity | $ 250,000,000 |  |  |  |  |  |  |  |  |  |  |  |
Senior Notes | 2.75% Senior Debt Obligations Due 2022 |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument, amount |  |  |  |  |  |  |  |  |  |  |  |  | $ 500,000,000
Interest rate |  |  |  |  |  |  |  |  |  |  |  |  | 2.75%
Senior Notes | 4.35% Senior Debt Obligations Due 2047 |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument, amount |  |  |  |  |  |  |  |  |  |  |  |  | $ 500,000,000
Interest rate |  |  |  |  |  |  |  |  |  |  |  |  | 4.35%
Jardine Lloyd Thompson Group plc | Bridge Loan |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative, notional amount | £ |  |  |  |  |  |  |  |  |  | £ 5,200 |  |  |
Payments of debt issuance costs |  | $ 35,000,000 |  |  |  |  |  |  |  |  |  |  |
Amortization of debt issuance costs |  | $ 30,000,000 |  |  |  |  |  |  |  |  |  |  |
Subsequent Event | Bridge Loan |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Decrease in debt | £ |  |  | £ (3,790) |  |  |  |  |  |  |  |  |  |
Subsequent Event | Senior Notes |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument, amount |  |  |  |  |  |  | $ 5,000,000,000 |  |  |  |  |  |
Subsequent Event | Senior Notes | 3.50% Senior Debt Obligations Due 2020 |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument, amount |  |  |  |  |  |  | $ 700,000,000 |  |  |  |  |  |
Interest rate |  |  |  |  |  |  | 3.50% |  |  |  |  |  |
Subsequent Event | Senior Notes | 3.875% Senior Debt Obligations Due 2024 |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument, amount |  |  |  |  |  |  | $ 1,000,000,000.0 |  |  |  |  |  |
Interest rate |  |  |  |  |  |  | 3.875% |  |  |  |  |  |
Subsequent Event | Senior Notes | 4.375% Senior Debt Obligations Due 2029 |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument, amount |  |  |  |  |  |  | $ 1,250,000,000.00 |  |  |  |  |  |
Interest rate |  |  |  |  |  |  | 4.375% |  |  |  |  |  |
Subsequent Event | Senior Notes | 4.75% Senior Debt Obligations Due 2039 |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument, amount |  |  |  |  |  |  | $ 500,000,000 |  |  |  |  |  |
Interest rate |  |  |  |  |  |  | 4.75% |  |  |  |  |  |
Subsequent Event | Senior Notes | 4.90% Senior Debt Obligations Due 2049 |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument, amount |  |  |  |  |  |  | $ 1,250,000,000.00 |  |  |  |  |  |
Interest rate |  |  |  |  |  |  | 4.90% |  |  |  |  |  |
Subsequent Event | Senior Notes | Floating Rate Senior Notes Due 2021 |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument, amount |  |  |  |  |  |  | $ 300,000,000 |  |  |  |  |  |

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