Borrowings (Narrative) (Details)		12 Months Ended
	Feb. 27, 2019 USD ($) credit_increase	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Line of Credit Facility [Line Items]
Limit on annual cash dividends paid		$ 30,000,000
Interest income		789,000	$ 784,000	$ 807,000
Interest expense		1,188,000	919,000	$ 1,282,000
Revolving Credit Facility
Line of Credit Facility [Line Items]
Outstanding borrowings		25,000,000	$ 0
Credit Agreement | Revolving Credit Facility
Line of Credit Facility [Line Items]
Credit Agreement, maximum borrowing capacity		$ 150,000,000
Credit Agreement term		5 years
Subsequent Event | Revolving Credit Facility
Line of Credit Facility [Line Items]
Credit Agreement, maximum borrowing capacity	$ 100,000,000
Credit Agreement term	5 years
Number of credit increases | credit_increase	4
Subsequent Event | Term Loan Facility
Line of Credit Facility [Line Items]
Credit Agreement, maximum borrowing capacity	$ 100,000,000
Credit Agreement term	5 years