Debt (Narrative) (Details) - USD ($)				3 Months Ended
	Feb. 25, 2021	Aug. 26, 2020	Aug. 08, 2019	Mar. 31, 2021	Mar. 31, 2020	Aug. 08, 2026	Aug. 08, 2025	Aug. 08, 2024	Dec. 31, 2020	May 22, 2020	Dec. 15, 2015	Jul. 30, 2015
Debt Instrument [Line Items]
Document Period End Date				Mar. 31, 2021
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	$ 600,000	$ 100,000	200,000	$ 595,000	$ 0
Costs associated with amendment				1,191,000					$ 1,313,000
Borrowings on lines of credit				3,809,000					106,687,000
Line of Credit Facility, Remaining Borrowing Capacity				32,300,000
Senior Notes			$ 75,000,000.0	235,561,000					111,056,000
Letters of Credit Outstanding, Amount				10,200,000					9,800,000
Proceeds from Unsecured Notes Payable	124,400,000	37,400,000		125,000,000	$ 0
Unsecured Debt	$ 125,000,000.0	$ 37,500,000
Unsecured Debt [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity				275,000,000.0
Borrowings on lines of credit											$ 265,000,000.0
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity				35,000,000.0						$ 75.0		$ 35,000,000.0
Borrowings on lines of credit				$ 5,000,000					$ 7,000,000
Line of Credit Facility, Interest Rate at Period End				4.00%
Letters of Credit Outstanding, Amount				$ 2,700,000
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