Executive Stock Plans And Stock-Based Compensation (Summary Of RSU, Restricted Stock, And PSA Activity) (Details) - $ / shares shares in Thousands	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted, Shares	72,032	74,776	80,932
Vested, Shares	(60,751)	(51,544)	(42,384)
Forfeited, Shares	0	(539)
Unvested at end of year, Shares	336,919	325,638	302,945	264,397
Granted, Weighted- Average Grant Date Fair Value	$ 64.13	$ 55.27	$ 51.62
Vested, Weighted- Average Grant Date Fair Value	45.51	44.57	41.64
Forfeited, Weighted- Average Grant Date Fair Value	0.00	55.63
Unvested at end of year, Weighted- Average Grant Date Fair Value	$ 55.77	$ 52.01	$ 49.95	$ 48.10
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted, Shares	0	0	0
Vested, Shares	0	(17,900)	0
Forfeited, Shares	0	0
Unvested at end of year, Shares	30,200	30,200	48,100	48,100
Granted, Weighted- Average Grant Date Fair Value	$ 0.00	$ 0.00	$ 0.00
Vested, Weighted- Average Grant Date Fair Value	0.00	42.26	0.00
Forfeited, Weighted- Average Grant Date Fair Value	0.00	0.00
Unvested at end of year, Weighted- Average Grant Date Fair Value	$ 42.37	$ 42.37	$ 42.33	$ 42.33
Performance Share Awards Psas [Member
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted, Shares	39,100	58,805	86,212
Vested, Shares	(41,667)	(52,230)	(85,387)
Forfeited, Shares	(6,783)	(525)
Unvested at end of year, Shares	151,000	160,350	154,300	153,475
Granted, Weighted- Average Grant Date Fair Value	$ 60.37	$ 49.17	$ 45.06
Vested, Weighted- Average Grant Date Fair Value	46.41	53.56	38.14
Forfeited, Weighted- Average Grant Date Fair Value	46.41	49.17
Unvested at end of year, Weighted- Average Grant Date Fair Value	$ 50.50	$ 46.86	$ 48.13	$ 45.58