Debt (Narratives) (Details) - USD ($)	3 Months Ended	6 Months Ended
	Sep. 30, 2019	Sep. 30, 2019	Mar. 31, 2019
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	280,463,000	280,463,000
Debt Instrument, Periodic Payment, Principal	20,000,000	30,000,000
Current portion of long term debt	65,000,000	65,000,000	$ 65,000,000
Deferred Costs			14,690,000
Accumulated Amortization, Deferred Finance Costs	5,596,000	5,596,000	4,547,000
Unsecured credit lines	2,398,000	2,398,000
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	18,887,000	27,774,000
Current portion of long term debt	65,000,000	65,000,000
Required payments [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Periodic Payment, Principal	1,113,000	2,226,000
Current portion of long term debt	4,450,000	4,450,000
Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Deferred Costs			2,789,000
Accumulated Amortization, Deferred Finance Costs	1,488,000	1,488,000	$ 1,209,000
New Revolving Credit Facility
Line of Credit Facility [Line Items]
Outstanding letters of credit	15,821,000	15,821,000
New Revolving Credit Facility | Standby Letters of Credit
Line of Credit Facility [Line Items]
Outstanding letters of credit	15,687,000	15,687,000
New Revolving Credit Facility | Letter of Credit
Line of Credit Facility [Line Items]
Outstanding letters of credit	134,000	134,000
New Revolving Credit Facility | Unsecured Lines of Credit
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	13,463,000	13,463,000
Outstanding borrowings	$ 9,162,000	$ 9,162,000