Fair Value Measurements and Derivative Instruments (Narrative) (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Investment securities at a loss position for more than 12 months, aggregate unrealized holding losses (less than at December 31, 2016)	$ 4,000,000	$ 2,000,000
Investment securities at a loss position for more than 12 months, estimated fair value	140,000,000	116,000,000
Investment securities at a loss position for less than 12 months, aggregate unrealized holding losses	2,000,000	1,000,000
Investment securities at a loss position for less than 12 months, estimated fair value	147,000,000	91,000,000
Amount of ineffectiveness	0
Term Loan | Notes due December 2020
Derivative [Line Items]
Loan amount	300,000,000
Foreign exchange contracts
Derivative [Line Items]
Derivative notional amount	10,000,000	$ 10,000,000
Interest rate swaps | Cash Flow Hedge
Derivative [Line Items]
Derivative notional amount	$ 300,000,000
Fixed interest rate	0.8826%