Bank Loans (Details) - Wells Fargo Credit Facility [Member] - USD ($)	6 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Line of Credit Facility [Line Items]
Line of credit, expiration date	Jan. 10, 2022
Line of credit, borrowing limit	$ 250,000,000.0
Line of credit, frequency of interest payment	monthly
Spread over LIBOR	0.825%
Line of credit, facility fee percent	0.125%
Line of credit, amount outstanding	$ 0	$ 0
Unamortized commitment fees	$ 576,000	$ 691,000
Maximum [Member]
Line of Credit Facility [Line Items]
Spread over LIBOR	1.55%
Line of credit, facility fee percent	0.30%
Minimum [Member]
Line of Credit Facility [Line Items]
Spread over LIBOR	0.80%
Line of credit, facility fee percent	0.10%