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Financial Instruments - (Narratives) (Details)	3 Months Ended
Financial Instruments - (Narratives) (Details)	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2018 EUR (€)	Sep. 30, 2017 EUR (€)	Sep. 30, 2017 GBP (£)
Senior notes \| Senior Euro Notes due January 2021, interest payable annually at 0.400% (2021 Euro Notes)
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Principal amount of debt \| €					€ 500,000,000
Senior notes \| Senior Euro Notes due July 2024, interest payable annually at 1.100% (2024 Euro Notes)
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Principal amount of debt \| €					€ 500,000,000
Senior notes \| Senior GBP Notes due June 2022, interest payable annually at 1.700% (2022 GBP Notes)
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Principal amount of debt \| £						£ 300,000,000
Interest rate swap
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Notional amount	$ 0		$ 0
Interest rate swap \| Fair Value Hedging
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Notional amount \| €				€ 500,000,000
Interest rate swap \| Fair Value Hedging \| Senior notes \| Senior Euro Notes due July 2024, interest payable annually at 1.100% (2024 Euro Notes)
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Derivative asset fair value	7,000,000
Interest rate swap \| Net investment hedging
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Notional amount			$ 716,000,000
Derivative liability fair value	3,000,000
Currency forward contract \| Net investment hedging
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Amount of gains reclassified from AOCE into income	$ 7,000,000	$ (38,000,000)