Fair Value Measurements - Hierarchy and Transfers (Details) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
	Jul. 29, 2017 USD ($)	Jan. 27, 2018 USD ($) company	Apr. 29, 2017 USD ($)
Assets
Total assets		$ 61,044	$ 53,677
Cost basis investments
Liabilities
Preferred share investments, number of privately-held companies | company		2
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		4,581	3,083
Level 1 | Recurring basis | Available-for-sale investments
Assets
Total assets		1,223	1,217
Level 1 | Recurring basis | Held-to-maturity investments
Assets
Total assets		3,358	1,866
Level 2 | Recurring basis
Assets
Total assets		38,183	36,644
Level 2 | Recurring basis | Available-for-sale investments
Assets
Total assets		38,171	36,638
Level 2 | Recurring basis | Trading securities
Assets
Total assets		$ 12	6
Level 3
Liabilities
Preferred share investments, number of privately-held companies | company		2
Preferred share investments with common share warrant, number of privately-held companies | company		1
Available-for-sale securities converted to cost basis investment	$ 3,000
Level 3 | Recurring basis
Assets
Total assets		$ 10,954	6,900
Liabilities
Contingent consideration liability		354	1,248
Level 3 | Recurring basis | Available-for-sale investments
Assets
Total assets			1,400
Level 3 | Recurring basis | Cost basis investments
Assets
Total assets		$ 10,954	$ 5,500