Collaboration and License Agreements - Nationwide Children's Hospital (Details)	1 Months Ended			12 Months Ended
	May 31, 2015 shares	Mar. 31, 2015 shares	Oct. 31, 2013 USD ($) installment $ / shares shares	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($) shares	Jun. 26, 2017 $ / shares	May 29, 2015 USD ($)
Collaboration and License Agreements
Share price (in dollars per share) | $ / shares							$ 70.00
Research and Development Expense				$ 150,391,000	$ 58,892,000	$ 27,493,000
Collaborative Arrangement | NCH
Collaboration and License Agreements
Minimum commitment to spend			$ 9,400,000
Minimum commitment to spend, period ( in years)			8 years
Shares issued (in shares) | shares	3,802	34,463	331,053
Ownership percentage (as a percent)			3.00%
Period following FDA approval of BLA to sell shares (in days)			30 days
Minimum ownership percentage of shares issued pursuant to the agreement (as a percent)			50.00%
Quarterly installments, number | installment			4
License patent expiration period (in years)			10 years
Market capitalization, achieved								$ 100,000,000
Period of prior written notice to terminate agreement			6 months
Termination, event of a material uncured breach, notice period (in days)			30 days
Research and development expenses | Collaborative Arrangement | NCH
Collaboration and License Agreements
Fair value of shares granted						473,164
Research and Development Expense						$ 100,000
Class B-1 Preferred Stock
Collaboration and License Agreements
Shares issued (in shares) | shares						1,012,000
Class B-1 Preferred Stock | Collaborative Arrangement | NCH
Collaboration and License Agreements
Ratio of the per share sale price to the price per share of preferred stock sold in Class B Financing			2
Share price (in dollars per share) | $ / shares			$ 2.47