Debt (Narrative) (Details) - USD ($)			3 Months Ended		9 Months Ended
	Feb. 25, 2021	Aug. 26, 2020	Sep. 30, 2021	Jun. 30, 2021	Sep. 30, 2021	Sep. 30, 2020	Aug. 08, 2026	Aug. 08, 2025	Aug. 08, 2024	Dec. 31, 2020	May 22, 2020	Aug. 08, 2019	Dec. 15, 2015	Jul. 30, 2015
Debt Instrument [Line Items]
Document Period End Date					Sep. 30, 2021
Debt Instrument, Interest Rate, Stated Percentage	3.25%											4.00%
Proceeds from Issuance of Senior Long-term Debt					$ 125,000,000	$ 37,500,000
Payments of Debt Issuance Costs	$ 600,000	$ 0.1		$ 300,000	894,000	$ 62,000
Costs associated with amendment			$ 1,283,000		1,283,000					$ 1,313,000
Borrowings on lines of credit			122,717,000		122,717,000					106,687,000
Line of Credit Facility, Remaining Borrowing Capacity			33,600,000		33,600,000
Senior Notes			235,737,000		235,737,000					111,056,000
Letters of Credit Outstanding, Amount			3,800,000		$ 3,800,000					9,800,000
Proceeds from Unsecured Notes Payable			$ 37.4	$ 124,400,000
Unsecured Debt	$ 125,000,000	$ 37,500,000
Unsecured Debt [Member]
Debt Instrument [Line Items]
Borrowings on lines of credit													$ 265,000,000
Unsecured Debt [Member] | Minimum [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Interest Rate at Period End			2.58%		2.58%
Unsecured Debt [Member] | Maximum [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Interest Rate at Period End			2.59%		2.59%
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity			$ 35,000,000		$ 35,000,000						$ 75		$ 275,000,000	$ 35,000,000
Borrowings on lines of credit			$ 19,000,000		$ 19,000,000					$ 7,000,000
Line of Credit Facility, Interest Rate at Period End			4.00%		4.00%
Letters of Credit Outstanding, Amount			$ 1,400,000		$ 1,400,000
Forecast [Member]
Debt Instrument [Line Items]
Long-term Debt, Maturities, Repayments of Principal in Rolling after Year Five							$ 50,000,000	$ 12,500,000	$ 12,500,000