Intangible Assets and Goodwill (Details) - USD ($)	1 Months Ended	3 Months Ended					12 Months Ended
	Dec. 31, 2018	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Oct. 10, 2019
Indefinite-Lived Intangible Assets [Line Items]
Intangible asset	$ 0	$ 0					$ 400,000,000.0	$ 0
Intangible asset impairment charges		29,000,000	$ 0	$ 0	$ 0		0	29,000,000	$ 255,340,000
Goodwill	50,384,000	$ 50,384,000					1,002,158,000	50,384,000
Semma Therapeutics, Inc.
Indefinite-Lived Intangible Assets [Line Items]
Intangible asset							387,000,000.0			$ 387,000,000
Goodwill							554,600,000			$ 554,633,000
Exonics Therapeutics
Indefinite-Lived Intangible Assets [Line Items]
Intangible asset							13,000,000.0
Goodwill							$ 397,100,000
Other Intangible Assets | Variable Interest Entity, Primary Beneficiary | BioAxone Biosciences, Inc
Indefinite-Lived Intangible Assets [Line Items]
Intangible asset impairment charges	$ 29,000,000.0							$ 29,000,000.0	29,000,000
Other Intangible Assets | Variable Interest Entity, Primary Beneficiary | Parion Sciences, Inc
Indefinite-Lived Intangible Assets [Line Items]
Intangible asset impairment charges						$ 255,300,000			$ 255,300,000