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Debt - Summary of outstanding debt (Details) | Dec. 31, 2018 USD ($) | Dec. 31, 2018 CAD ($) | Dec. 31, 2018 EUR (€) | Dec. 03, 2018 USD ($) | Mar. 08, 2018 USD ($) | Mar. 08, 2018 CAD ($) | Dec. 31, 2017 USD ($)
--- | --- | --- | --- | --- | --- | --- | ---
Debt Instrument [Line Items] |  |  |  |  |  |  |
Total debt | $ 6,244,000,000 |  |  |  |  |  | $ 5,966,000,000
Less: Short-term and current portion of long-term debt | 251,000,000 |  |  |  |  |  | 299,000,000
Long-term debt | 5,993,000,000 |  |  |  |  |  | 5,667,000,000
Commercial Paper |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Total debt | 250,000,000 |  |  |  |  |  | 0
Other |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Total debt | $ 1,000,000 |  |  |  |  |  | 3,000,000
3.875% Senior Notes due December 2025 | Senior Notes |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Interest rate on debt | 3.875% | 3.875% | 3.875% |  |  |  |
Total debt | $ 746,000,000 |  |  |  |  |  | 745,000,000
5.00% Senior Notes due September 2020 | Senior Notes |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Interest rate on debt | 5.00% | 5.00% | 5.00% |  |  |  |
Total debt | $ 599,000,000 |  |  |  |  |  | 598,000,000
3.50% Senior Notes due June 2024 | Senior Notes |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Interest rate on debt | 3.50% | 3.50% | 3.50% |  |  |  |
Total debt | $ 596,000,000 |  |  |  |  |  | 595,000,000
4.75% Senior Notes due May 2045 | Senior Notes |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Interest rate on debt | 4.75% | 4.75% | 4.75% |  |  |  |
Total debt | $ 592,000,000 |  |  |  |  |  | 592,000,000
2.875% Senior Notes due May 2026 (EUR 500M) | Senior Notes |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Interest rate on debt | 2.875% | 2.875% | 2.875% |  |  |  |
Debt face value | € |  |  | € 500,000,000 |  |  |  |
Total debt | $ 562,000,000 |  |  |  |  |  | 587,000,000
4.60% Senior Notes due June 2044 | Senior Notes |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Interest rate on debt | 4.60% | 4.60% | 4.60% |  |  |  |
Total debt | $ 544,000,000 |  |  |  |  |  | 544,000,000
8.205% Junior Subordinated Notes due January 2027 | Senior Notes |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Interest rate on debt | 8.205% | 8.205% | 8.205% |  |  |  |
Total debt | $ 521,000,000 |  |  |  |  |  | 521,000,000
2.80% Senior Notes due March 2021 | Senior Notes |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Interest rate on debt | 2.80% | 2.80% | 2.80% |  |  |  |
Total debt | $ 398,000,000 |  |  |  |  |  | 398,000,000
4.00% Senior Notes due November 2023 | Senior Notes |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Interest rate on debt | 4.00% | 4.00% | 4.00% |  |  |  |
Total debt | $ 348,000,000 |  |  |  |  |  | 348,000,000
4.500% Senior Notes due December 2028 | Senior Notes |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Interest rate on debt | 4.50% | 4.50% | 4.50% | 4.50% |  |  |
Debt face value |  |  |  | $ 350,000,000 |  |  |
Total debt | $ 347,000,000 |  |  |  |  |  | 0
6.25% Senior Notes due September 2040 | Senior Notes |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Interest rate on debt | 6.25% | 6.25% | 6.25% |  |  |  |
Total debt | $ 296,000,000 |  |  |  |  |  | 296,000,000
4.76% Senior Notes due March 2018 (CAD 375M) | Senior Notes |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Interest rate on debt | 4.76% | 4.76% | 4.76% |  | 4.76% | 4.76% |
Debt face value |  | $ 375,000,000 |  |  |  |  |
Total debt | $ 0 |  |  |  |  |  | 296,000,000
Less: Short-term and current portion of long-term debt |  |  |  |  | $ 291,000,000 | $ 375,000,000 |
4.45% Senior Notes due May 2043 | Senior Notes |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Interest rate on debt | 4.45% | 4.45% | 4.45% |  |  |  |
Total debt | $ 246,000,000 |  |  |  |  |  | 246,000,000
4.25% Senior Notes due December 2042 | Senior Notes |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Interest rate on debt | 4.25% | 4.25% | 4.25% |  |  |  |
Total debt | $ 198,000,000 |  |  |  |  |  | $ 197,000,000

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