Debt (Schedule of Lines of Credit Outstanding) (Details) - USD ($)	Jun. 30, 2019	Dec. 31, 2018	Dec. 15, 2015	Jul. 30, 2015
Line of Credit Facility [Line Items]
Long-term Line of Credit	$ 232,657,000	$ 200,386,000
Debt issuance costs, net of amortization	(1,343,000)	(1,614,000)
Letters of Credit Outstanding, Amount	2,000,000.0	2,200,000
Line of Credit Facility, Remaining Borrowing Capacity	73,100,000
Unsecured Debt [Member]
Line of Credit Facility [Line Items]
Long-term Line of Credit	215,000,000.0		$ 40,000,000.0
Line of Credit Facility, Maximum Borrowing Capacity	275,000,000.0
Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Long-term Line of Credit	55,500,000	46,500,000
Line of Credit Facility, Maximum Borrowing Capacity	75,000,000.0			$ 50,000,000.0
Letters of Credit Outstanding, Amount	1,900,000
Unsecured Debt [Member]
Line of Credit Facility [Line Items]
Long-term Line of Credit	$ 178,500,000	$ 155,500,000