Stock-based compensation - Stock option activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Mar. 28, 2020	Mar. 30, 2019	Mar. 31, 2018
Shares
Beginning balance (in shares)	2,895,539	3,040,206	2,946,028
Granted (in shares)			343,352
Forfeited (in shares)	(42,864)	(27,793)	(90,881)
Expired (in shares)	(293,443)	(116,874)	(158,293)
Ending balance (in shares)	2,559,232	2,895,539	3,040,206
Vested and exercisable at end of year (in shares)	2,389,873	2,428,274	2,241,283
Weighted-average exercise price
Balance at the beginning of the period (in dollars per share)	$ 15.27	$ 15.40	$ 16.81
Granted (in dollars per share)			4.10
Forfeited (in dollars per share)	8.83	18.00	15.13
Expired (in dollars per share)	15.98	17.89	17.31
Balance at the end of the period (in dollars per share)	15.30	15.27	15.40
Exercisable at the end of the period (in dollars per share)	$ 15.69	$ 16.49	$ 17.53
Weighted-average contractual term remaining
Balance at end of year	4 years 3 months 7 days	5 years 3 months 7 days	6 years 2 months 23 days
Exercisable at the end of the period	4 years 1 month 28 days	4 years 11 months 12 days	5 years 7 months 24 days
Aggregate intrinsic value
Balance at the end of the period		$ 2,460,384	$ 482
Exercisable at the end of the period		$ 1,102,326	$ 7