Revolving Credit Facility (Narrative) (Details) $ in Millions	6 Months Ended
	Jun. 30, 2018 USD ($)
Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 15.0
Credit facility, amount outstanding	$ 0.0
Line of credit facility, unused capacity, commitment fee percentage	0.25%
Revolving Credit Facility [Member] | London Interbank Offered Rate (LIBOR) [Member]
Line of Credit Facility [Line Items]
Line of credit facility, interest rate during period	2.50%
Revolving Credit Facility B [Member]
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 30.0
Credit facility, amount outstanding	$ 30.0
Line of credit facility, unused capacity, commitment fee percentage	0.25%
Revolving Credit Facility B [Member] | London Interbank Offered Rate (LIBOR) [Member]
Line of Credit Facility [Line Items]
Line of credit facility, unused capacity, commitment fee percentage	3.00%
Letter of Credit [Member]
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 5.0
Line of credit facility, commitment fee percentage	1.00%