Fair Value Measurements - Hierarchy and Transfers (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Jan. 26, 2019	Apr. 28, 2018
Assets
Total assets	$ 57,669	$ 59,354
Recurring basis
Liabilities
Transfers from Level 1 to Level 2 assets	0	0
Transfers from Level 2 to Level 1 assets	0	0
Transfers from Level 1 to Level 2 liabilities	0	0
Transfers from Level 2 to Level 1 liabilities	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
Recurring basis | Level 1
Assets
Marketable securities	163	1,141
Held-to-maturity investments	3,384	3,340
Total assets	3,547	4,481
Recurring basis | Level 2
Assets
Marketable securities	34,852	37,173
Total assets	34,852	37,173
Recurring basis | Level 3
Assets
Cost basis investments	11,979	10,954
Total assets	11,979	10,954
Liabilities
Contingent consideration liability	$ 7,500	$ 344