Debt - Summary of outstanding debt (Details)	Dec. 31, 2017 USD ($)	Dec. 31, 2017 EUR (€)	Dec. 31, 2017 CAD	Dec. 31, 2016 USD ($)	Mar. 01, 2016 USD ($)
Debt Instrument [Line Items]
Total debt	$ 5,966,000,000			$ 6,205,000,000
Less: Short-term and current portion of long-term debt	299,000,000			336,000,000
Long-term debt	5,667,000,000			5,869,000,000
Commercial Paper
Debt Instrument [Line Items]
Total debt	0			329,000,000
Other
Debt Instrument [Line Items]
Total debt	3,000,000			9,000,000
3.875% Senior Notes due December 2025 | Senior Notes
Debt Instrument [Line Items]
Total debt	$ 745,000,000			744,000,000
Interest rate on debt	3.875%	3.875%	3.875%
Debt face value					$ 750,000,000
5.00% Senior Notes due September 2020 | Senior Notes
Debt Instrument [Line Items]
Total debt	$ 598,000,000			598,000,000
Interest rate on debt	5.00%	5.00%	5.00%
3.50% Senior Notes due June 2024 | Senior Notes
Debt Instrument [Line Items]
Total debt	$ 592,000,000			594,000,000
Interest rate on debt	3.50%	3.50%	3.50%
4.75% Senior Notes due May 2045 | Senior Notes
Debt Instrument [Line Items]
Total debt	$ 595,000,000			592,000,000
Interest rate on debt	4.75%	4.75%	4.75%
2.875% Senior Notes due May 2026 (EUR 500M) | Senior Notes
Debt Instrument [Line Items]
Total debt	$ 544,000,000			516,000,000
Interest rate on debt	2.875%	2.875%	2.875%
Debt face value | €		€ 500,000,000
4.60% Senior Notes due June 2044 | Senior Notes
Debt Instrument [Line Items]
Total debt	$ 587,000,000			543,000,000
Interest rate on debt	4.60%	4.60%	4.60%
8.205% Junior Subordinated Notes due January 2027 | Senior Notes
Debt Instrument [Line Items]
Total debt	$ 521,000,000			521,000,000
Interest rate on debt	8.205%	8.205%	8.205%
2.80% Senior Notes due March 2021 | Senior Notes
Debt Instrument [Line Items]
Total debt	$ 398,000,000			397,000,000
Interest rate on debt	2.80%	2.80%	2.80%
4.00% Senior Notes due November 2023 | Senior Notes
Debt Instrument [Line Items]
Total debt	$ 348,000,000			347,000,000
Interest rate on debt	4.00%	4.00%	4.00%
6.25% Senior Notes due September 2040 | Senior Notes
Debt Instrument [Line Items]
Total debt	$ 296,000,000			295,000,000
Interest rate on debt	6.25%	6.25%	6.25%
4.76% Senior Notes due March 2018 (CAD 375M) | Senior Notes
Debt Instrument [Line Items]
Total debt	$ 296,000,000			277,000,000
Interest rate on debt	4.76%	4.76%	4.76%
Debt face value | CAD			CAD 375,000,000
4.45% Senior Notes due May 2043 | Senior Notes
Debt Instrument [Line Items]
Total debt	$ 246,000,000			246,000,000
Interest rate on debt	4.45%	4.45%	4.45%
4.25% Senior Notes due December 2042 | Senior Notes
Debt Instrument [Line Items]
Total debt	$ 197,000,000			$ 197,000,000
Interest rate on debt	4.25%	4.25%	4.25%