Bank Loans (Narrative) (Details) - USD ($)	6 Months Ended
	Jun. 30, 2020	Dec. 31, 2019
Line of Credit Facility [Line Items]
Credit facility, outstanding	$ 0	$ 0
Wells Fargo Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of credit, borrowing limit	$ 250,000,000.0
Line of credit, expiration date	Jan. 10, 2022
Spread over LIBOR	0.825%
Line of credit, facility fee percent	0.125%
Unamortized loan origination costs	$ 345,000	$ 461,000
Line of credit, frequency of interest payment	monthly
Wells Fargo Credit Facility [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Spread over LIBOR	1.55%
Line of credit, facility fee percent	0.30%
Wells Fargo Credit Facility [Member] | Minimum [Member]
Line of Credit Facility [Line Items]
Spread over LIBOR	0.80%
Line of credit, facility fee percent	0.10%