Borrowings (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Line of Credit Facility [Line Items]
Interest income	$ 784	$ 807	$ 521
Interest expense	919	1,282	$ 2,011
Deferred tax liability on tax effect basis difference between carrying values and tax basis of the convertible notes	$ 642
Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Credit Agreement initiation date	Oct. 10, 2014
Credit Agreement term	5 years
Credit Agreement, maximum borrowing capacity	$ 150,000
Credit Agreement optional increase in aggregate principal amount	75,000
Outstanding borrowings	0	$ 0
Credit Agreement maximum cash dividends in a fiscal year	$ 30,000