Leases - Supplemental Cash Flow Information (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($)
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows from operating leases	$ 10,650
Operating cash flows from finance leases	50,527
Financing cash flows from finance leases	39,185
Right-of-use assets obtained in exchange for lease obligations
Operating leases	34,605
Finance leases	0
Semma and Exonics Therapeutics
Right-of-use assets obtained in exchange for lease obligations
Operating leases	$ 33,700