Revenues - Revenues by Disaggregated Category (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 28, 2019	Jun. 29, 2018	Jun. 28, 2019	Jun. 29, 2018
Disaggregation of Revenue [Line Items]
Total revenues	$ 240,275	$ 186,108	$ 455,762	$ 399,827
Collaborative Arrangement with Ipsen
Disaggregation of Revenue [Line Items]
Total revenues	22,249	34,043	$ 44,117	87,852
GSK \| Collaborative Arrangement with Ipsen
Disaggregation of Revenue [Line Items]
Percent of royalty on net sale			3.00%
Net product revenues
Disaggregation of Revenue [Line Items]
Total revenues	193,675	145,836	$ 373,256	280,108
Gross product revenues
Disaggregation of Revenue [Line Items]
Total revenues	240,418	172,646	464,168	332,082
Discounts and allowances
Disaggregation of Revenue [Line Items]
Total revenues	46,743	26,810	90,912	51,974
Total collaboration revenues
Disaggregation of Revenue [Line Items]
Total revenues	46,600	40,272	82,506	119,719
License revenues
Disaggregation of Revenue [Line Items]
Total revenues	36,393	31,908	60,902	100,938
Research and development services revenues
Disaggregation of Revenue [Line Items]
Total revenues	10,898	6,805	22,826	16,904
Other collaboration revenues
Disaggregation of Revenue [Line Items]
Total revenues	$ (691)	$ 1,559	$ (1,222)	$ 1,877