Debt (Narratives) (Details) - USD ($)	3 Months Ended	6 Months Ended
	Sep. 30, 2020	Sep. 30, 2020	Mar. 31, 2020
Line of Credit Facility [Line Items]
New Credit Agreement 2017	$ 545,000,000	$ 545,000,000
Maximum availability under New Revolving Credit Facility	100,000,000	100,000,000
Outstanding borrowings	257,125,000	257,125,000
Current portion of long term debt	29,450,000	29,450,000	$ 4,450,000
Deferred Costs	14,690,000	14,690,000
Accumulated Amortization, Deferred Finance Costs	7,695,000	7,695,000	6,645,000
Unsecured credit lines	2,578,000	2,578,000
Amount drawn on unsecured credit lines		0
Proceeds from Secured Lines of Credit		445,000,000
Line of Credit Facility, Fair Value of Amount Outstanding	25,000,000	25,000,000
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	4,450,000
Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Deferred Costs	3,615,000	3,615,000
Accumulated Amortization, Deferred Finance Costs	2,044,000	2,044,000	$ 1,766,000
New Revolving Credit Facility
Line of Credit Facility [Line Items]
Outstanding letters of credit	16,729,000	16,729,000
New Revolving Credit Facility | Standby Letters of Credit
Line of Credit Facility [Line Items]
Outstanding letters of credit	16,729,000	16,729,000
New Revolving Credit Facility | Letter of Credit
Line of Credit Facility [Line Items]
Outstanding letters of credit	0	0
New Revolving Credit Facility | Unsecured Lines of Credit
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	15,749,000	15,749,000
Outstanding borrowings	$ 12,048,000	$ 12,048,000