Fair Value Measurements - Hierarchy and Transfers (Details) - USD ($) $ in Thousands	12 Months Ended
	Apr. 29, 2017	Apr. 30, 2016
Assets
Total assets	$ 53,677	$ 46,976
Recurring basis
Liabilities
Transfers from Level 1 to Level 2 assets	0	0
Transfers from Level 2 to Level 1 assets	0	0
Transfers into Level 3 assets	0	0
Transfers out of Level 3 assets	0	0
Transfers into Level 3 liabilities	0	0
Transfers out of Level 3 liabilities	0	0
Level 1 | Recurring basis
Assets
Total assets	3,083	3,003
Level 1 | Recurring basis | Available-for-sale investments
Assets
Total assets	1,217	1,177
Level 1 | Recurring basis | Held-to-maturity investments
Assets
Total assets	1,866	1,826
Level 2 | Recurring basis
Assets
Total assets	36,644	36,802
Level 2 | Recurring basis | Available-for-sale investments
Assets
Total assets	36,638	$ 36,802
Level 2 | Recurring basis | Trading securities
Assets
Total assets	6
Level 3 | Recurring basis
Assets
Total assets	6,900
Liabilities
Contingent consideration liability	1,248
Level 3 | Recurring basis | Available-for-sale investments
Assets
Total assets	1,400
Level 3 | Recurring basis | Cost basis investment
Assets
Total assets	$ 5,500