Investments (Amortized Cost And Fair Value Of Securities Available-For-Sale) (Details) (USD $)	Jul. 31, 2013		Apr. 30, 2013		Jul. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Investments in available-for-sale securities, Amortized Cost	$ 493,565,000		$ 480,628,000		$ 375,545,000
Gross Unrealized Gains	4,091,000		6,912,000		6,324,000
Gross Unrealized Losses	(10,623,000)	[1]	(664,000)	[1]	(78,000)	[1]
Investments in available-for-sale securities, Fair Value	487,033,000		486,876,000		381,791,000
Mortgage-Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Mortgage-backed securities, cost					7,900,000
Mortgage-backed securities, gross unrealized loss					5,000
Short-Term | Municipal Bonds
Schedule of Available-for-sale Securities [Line Items]
Investments in available-for-sale securities, Amortized Cost					1,006,000
Gross Unrealized Gains					20,000
Gross Unrealized Losses					0	[1]
Investments in available-for-sale securities, Fair Value					1,026,000
Long-Term
Schedule of Available-for-sale Securities [Line Items]
Investments in available-for-sale securities, Amortized Cost					374,539,000
Gross Unrealized Gains					6,304,000
Gross Unrealized Losses					(78,000)	[1]
Investments in available-for-sale securities, Fair Value					380,765,000
Long-Term | Mortgage-Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Investments in available-for-sale securities, Amortized Cost	489,401,000		476,450,000		370,318,000
Gross Unrealized Gains	3,825,000		6,592,000		5,898,000
Gross Unrealized Losses	(10,623,000)	[1]	(664,000)	[1]	(78,000)	[1]
Investments in available-for-sale securities, Fair Value	482,603,000		482,378,000		376,138,000
Long-Term | Municipal Bonds
Schedule of Available-for-sale Securities [Line Items]
Investments in available-for-sale securities, Amortized Cost	4,164,000		4,178,000		4,221,000
Gross Unrealized Gains	266,000		320,000		406,000
Gross Unrealized Losses	0	[1]	0	[1]	0	[1]
Investments in available-for-sale securities, Fair Value	$ 4,430,000		$ 4,498,000		$ 4,627,000

[1] As of July 31, 2013 and April 30, 2013, we had no securities that had been in a continuous loss position for more than twelve months. As of July 31, 2012, mortgage-backed securities with a cost of $7.9 million and gross unrealized losses of $5 thousand had been in a continuous loss position for more than twelve months.