Risk Management - Foreign Currency Risk (Details)	Mar. 31, 2018 USD ($) position	Mar. 31, 2018 GBP (£) position	Dec. 31, 2017 USD ($)	Dec. 31, 2016 position
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Number of net notional positions to buy or sell foreign currency disclosed (in number of positions) | position	5	5		5
Derivative [Line Items]
Fair value of derivative liabilities	$ 25,000,000
Foreign Exchange Contract
Derivative [Line Items]
Notional amounts of outstanding foreign exchange contracts	643,000,000		$ 507,000,000
Foreign Exchange Contract | Euro | Long
Derivative [Line Items]
Notional amounts of outstanding foreign exchange contracts	238,000,000		149,000,000
Foreign Exchange Contract | British Pound | Designated as Hedging Instrument | Net Investment Hedging
Derivative [Line Items]
Fair value of derivative liabilities	7,000,000
Foreign Exchange Contract | British Pound | Designated as Hedging Instrument | Net Investment Hedging | Derivative Contract Expiring June 2018
Derivative [Line Items]
Notional amounts of outstanding foreign exchange contracts | £		£ 25,000,000
Foreign Exchange Contract | British Pound | Designated as Hedging Instrument | Net Investment Hedging | Derivative Contract Expiring September 2018
Derivative [Line Items]
Notional amounts of outstanding foreign exchange contracts | £		£ 25,000,000
Foreign Exchange Contract | British Pound | Long
Derivative [Line Items]
Notional amounts of outstanding foreign exchange contracts	103,000,000		72,000,000
Foreign Exchange Contract | Chinese Renminbi | Short
Derivative [Line Items]
Notional amounts of outstanding foreign exchange contracts	72,000,000		73,000,000
Foreign Exchange Contract | Brazilian Real | Short
Derivative [Line Items]
Notional amounts of outstanding foreign exchange contracts	45,000,000		45,000,000
Foreign Exchange Contract | Australian Dollar | Short
Derivative [Line Items]
Notional amounts of outstanding foreign exchange contracts	$ 42,000,000		$ 64,000,000