Debt (Narrative) (Details) - USD ($)		9 Months Ended
	Aug. 08, 2019	Sep. 30, 2019	Sep. 30, 2018	Aug. 08, 2026	Aug. 08, 2025	Aug. 08, 2024	Jul. 23, 2019	Dec. 31, 2018	Dec. 15, 2015	Jul. 30, 2015
Debt Instrument [Line Items]
Document Period End Date		Sep. 30, 2019
Debt Instrument, Interest Rate, Stated Percentage	4.00%
Proceeds from Issuance of Senior Long-term Debt	$ 73,300,000	$ 75,000,000	$ 0
Debt Instrument, Fee Amount	1,500,000
Payments of Debt Issuance Costs	200,000	1,682,000	228,000
Repayments of Lines of Credit		201,500,000	$ 40,000,000
Letters of Credit Issued							$ 1,300,000
Costs associated with amendment		1,208,000						$ 1,614,000
Borrowings on lines of credit		164,792,000						200,386,000
Line of Credit Facility, Remaining Borrowing Capacity		71,800,000
Senior Notes	$ 75,000,000.0	73,358,000						0
Unsecured Debt [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		275,000,000.0
Borrowings on lines of credit		$ 215,000,000.0							$ 40,000,000.0
Unsecured Debt [Member] | Minimum [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Interest Rate at Period End		4.53%
Unsecured Debt [Member] | Maximum [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Interest Rate at Period End		4.55%
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 75,000,000.0								$ 50,000,000.0
Borrowings on lines of credit		$ 31,500,000						$ 46,500,000
Line of Credit Facility, Interest Rate at Period End		4.75%
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