Debt (Narratives) (Details) - USD ($)	3 Months Ended
	Jun. 30, 2019	Mar. 31, 2019
Line of Credit Facility [Line Items]
New Credit Agreement 2017	$ 545,000,000
Maximum availability under New Revolving Credit Facility	100,000,000
Outstanding borrowings	300,463,000
Current portion of long term debt	65,000,000	$ 65,000,000
Deferred Costs	14,690,000
Accumulated Amortization, Deferred Finance Costs	5,071,000	4,547,000
Unsecured credit lines	2,501,000
Amount drawn on unsecured credit lines	0
Proceeds from Secured Lines of Credit	445,000,000
Voluntary Debt Payment [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Periodic Payment, Principal	8,887,000
Current portion of long term debt	65,000,000
Required payments [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Periodic Payment, Principal	1,113,000
Current portion of long term debt	4,450,000
Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Deferred Costs	2,789,000
Accumulated Amortization, Deferred Finance Costs	1,348,000	$ 1,209,000
New Revolving Credit Facility
Line of Credit Facility [Line Items]
Outstanding letters of credit	16,325,000
New Revolving Credit Facility | Standby Letters of Credit
Line of Credit Facility [Line Items]
Outstanding letters of credit	15,879,000
New Revolving Credit Facility | Letter of Credit
Line of Credit Facility [Line Items]
Outstanding letters of credit	446,000
New Revolving Credit Facility | Unsecured Lines of Credit
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	14,036,000
Outstanding borrowings	$ 9,858,000