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