Revenue Recognition (Contract Balances) (Details) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2018 USD ($)
Changes in contract liabilities
Balance at Beginning of Period	$ 3,316	$ 2,048
Additions	2,434	8,605
Deductions	(3,544)	(8,447)
Balance at End of Period	2,206	2,206
Contract assets	0	0
Revenue recognized included in contract liabilities at the beginning of the period	$ 980	$ 2,365