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Fair Value Measurements Other (Details) - Significant unobservable inputs (Level 3) - Discounted cash flow - Auction rate securities
12 Months Ended
Apr. 30, 2016
Fair Value Inputs  
Coupon rates (as a percent) 0.10%
Minimum  
Fair Value Inputs  
Estimated redemption periods 3 years
Discount rates (as a percent) 4.90%
Maximum  
Fair Value Inputs  
Estimated redemption periods 18 years
Discount rates (as a percent) 15.10%