Debt (Schedule of Lines of Credit Outstanding) (Details) - USD ($)	3 Months Ended
	Mar. 31, 2021	Dec. 31, 2020	May 22, 2020	Dec. 15, 2015	Jul. 30, 2015
Line of Credit Facility [Line Items]
Document Period End Date	Mar. 31, 2021
Long-term Line of Credit	$ 3,809,000	$ 106,687,000
Debt issuance costs, net of amortization	(1,191,000)	(1,313,000)
Letters of Credit Outstanding, Amount	10,200,000	9,800,000
Line of Credit Facility, Remaining Borrowing Capacity	32,300,000
Unsecured Debt [Member]
Line of Credit Facility [Line Items]
Long-term Line of Credit				$ 265,000,000.0
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	5,000,000	7,000,000
Line of Credit Facility, Maximum Borrowing Capacity	35,000,000.0		$ 75.0		$ 35,000,000.0
Letters of Credit Outstanding, Amount	2,700,000
Unsecured Debt [Member]
Line of Credit Facility [Line Items]
Long-term Line of Credit	$ 0	$ 101,000,000