Debt - Summary of outstanding debt (Details) € in Millions, CAD in Millions	Dec. 31, 2016 USD ($)	Dec. 31, 2016 CAD	Dec. 31, 2016 EUR (€)	May 27, 2016 USD ($)	Mar. 01, 2016 USD ($)	Dec. 31, 2015 USD ($)	Nov. 13, 2015 USD ($)	May 20, 2015 USD ($)
Debt Instrument [Line Items]
Total debt	$ 6,205,000,000					$ 5,700,000,000
Less short-term and current portion of long-term debt	336,000,000					562,000,000
Long-term debt	5,869,000,000					5,138,000,000
Commercial Paper
Debt Instrument [Line Items]
Total debt	329,000,000					50,000,000
Other
Debt Instrument [Line Items]
Total debt	9,000,000					15,000,000
3.875% Senior Notes due December 2025 | Senior Notes
Debt Instrument [Line Items]
Total debt	$ 744,000,000					0
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					597,000,000
Interest rate on debt	5.00%	5.00%	5.00%
4.75% Senior Notes due May 2045 | Senior Notes
Debt Instrument [Line Items]
Total debt	$ 592,000,000					591,000,000
Interest rate on debt	4.75%	4.75%	4.75%
Debt face value								$ 600,000,000
3.50% Senior Notes due June 2024 | Senior Notes
Debt Instrument [Line Items]
Total debt	$ 594,000,000					593,000,000
Interest rate on debt	3.50%	3.50%	3.50%
4.60% Senior Notes due June 2044 | Senior Notes
Debt Instrument [Line Items]
Total debt	$ 543,000,000					543,000,000
Interest rate on debt	4.60%	4.60%	4.60%
2.875% Senior Notes due May 2026 (EUR 500M) | Senior Notes
Debt Instrument [Line Items]
Total debt	$ 516,000,000					541,000,000
Interest rate on debt	2.875%	2.875%	2.875%
Debt face value | €			€ 500
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%
3.125% Senior Notes due May 2016 | Senior Notes
Debt Instrument [Line Items]
Total debt	$ 0					500,000,000
Interest rate on debt	3.125%	3.125%	3.125%
Debt face value				$ 500,000,000
2.80% Senior Notes due March 2021 | Senior Notes
Debt Instrument [Line Items]
Total debt	$ 397,000,000					396,000,000
Interest rate on debt	2.80%	2.80%	2.80%
Debt face value							$ 400,000,000
4.00% Senior Notes due November 2023 | Senior Notes
Debt Instrument [Line Items]
Total debt	$ 347,000,000					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	$ 295,000,000					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	$ 277,000,000					270,000,000
Interest rate on debt	4.76%	4.76%	4.76%
Debt face value | CAD		CAD 375
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					$ 195,000,000
Interest rate on debt	4.25%	4.25%	4.25%