Debt (Narrative) (Details) - USD ($)		3 Months Ended	9 Months Ended
	Aug. 08, 2019	Sep. 30, 2020	Sep. 30, 2020	Sep. 30, 2019	Aug. 08, 2026	Aug. 08, 2025	Aug. 08, 2024	Aug. 26, 2020	May 22, 2020	Dec. 31, 2019	Dec. 15, 2015	Jul. 30, 2015
Debt Instrument [Line Items]
Document Period End Date			Sep. 30, 2020
Debt Instrument, Interest Rate, Stated Percentage	4.00%
Proceeds from Issuance of Senior Long-term Debt	$ 73,300,000
Debt Instrument, Fee Amount	1,500,000
Payments of Debt Issuance Costs	200,000		$ 62,000	$ 1,682,000
Costs associated with amendment		$ 1,011,000	1,011,000							$ 1,358,000
Borrowings on lines of credit		93,489,000	93,489,000							164,642,000
Line of Credit Facility, Remaining Borrowing Capacity		27,800,000	27,800,000
Senior Notes	$ 75,000,000.0	111,028,000	111,028,000							73,406,000
Letters of Credit Outstanding, Amount		7,200,000	7,200,000
Proceeds from Unsecured Notes Payable		37.4	37,500,000	$ 75,000,000
Unsecured Debt								$ 37,500,000
Unsecured Debt [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 275,000,000.0	$ 275,000,000.0
Borrowings on lines of credit											$ 215,000,000.0
Unsecured Debt [Member] | Minimum [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Interest Rate at Period End		2.64%	2.64%
Unsecured Debt [Member] | Maximum [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Interest Rate at Period End		2.66%	2.66%
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 35,000,000.0	$ 35,000,000.0						$ 75.0			$ 35,000,000.0
Borrowings on lines of credit		$ 4,000,000	$ 4,000,000							$ 38,000,000
Line of Credit Facility, Interest Rate at Period End		4.00%	4.00%
Letters of Credit Outstanding, Amount		$ 7,200,000	$ 7,200,000
Forecast [Member]
Debt Instrument [Line Items]
Long-term Debt, Maturities, Repayments of Principal in Rolling after Year Five					$ 50,000,000.0	$ 12,500,000	$ 12,500,000