Collaboration Agreements - Summary of Potential Success Payments, Payable in Cash or Publicly-Traded Equity (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Fred Hutchinson Cancer Research Center [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Per share common stock price required for payment	$ 60.66	$ 52.22
Fred Hutchinson Cancer Research Center [Member] | Series A convertible preferred stock [Member] | Range One [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Multiple of Equity Value at issuance	5.0
Per share common stock price required for payment	$ 20.00
Success payments	$ 10
Fred Hutchinson Cancer Research Center [Member] | Series A convertible preferred stock [Member] | Range Two [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Multiple of Equity Value at issuance	7.5
Per share common stock price required for payment	$ 30.00
Success payments	25
Fred Hutchinson Cancer Research Center [Member] | Series A convertible preferred stock [Member] | Range Three [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Multiple of Equity Value at issuance	10.0
Per share common stock price required for payment	$ 40.00
Success payments	40
Fred Hutchinson Cancer Research Center [Member] | Series A convertible preferred stock [Member] | Range Four [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Multiple of Equity Value at issuance	15.0
Per share common stock price required for payment	$ 60.00
Success payments	50
Fred Hutchinson Cancer Research Center [Member] | Series A convertible preferred stock [Member] | Range Five [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Multiple of Equity Value at issuance	20.0
Per share common stock price required for payment	$ 80.00
Success payments	50
Fred Hutchinson Cancer Research Center [Member] | Series A convertible preferred stock [Member] | Range Six [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Multiple of Equity Value at issuance	25.0
Per share common stock price required for payment	$ 100
Success payments	50
Fred Hutchinson Cancer Research Center [Member] | Series A convertible preferred stock [Member] | Range Seven [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Multiple of Equity Value at issuance	30.0
Per share common stock price required for payment	$ 120
Success payments	50
Fred Hutchinson Cancer Research Center [Member] | Series A convertible preferred stock [Member] | Range Eight [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Multiple of Equity Value at issuance	35.0
Per share common stock price required for payment	$ 140
Success payments	50
Fred Hutchinson Cancer Research Center [Member] | Series A convertible preferred stock [Member] | Range Nine [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Multiple of Equity Value at issuance	40.0
Per share common stock price required for payment	$ 160
Success payments	50
Memorial Sloan Kettering Cancer Center [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Per share common stock price required for payment	$ 60.66	$ 52.22
Memorial Sloan Kettering Cancer Center [Member] | Series A convertible preferred stock [Member] | Range One [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Multiple of Equity Value at issuance	10.0
Per share common stock price required for payment	$ 40.00
Success payments	10
Memorial Sloan Kettering Cancer Center [Member] | Series A convertible preferred stock [Member] | Range Two [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Multiple of Equity Value at issuance	15.0
Per share common stock price required for payment	$ 60.00
Success payments	70
Memorial Sloan Kettering Cancer Center [Member] | Series A convertible preferred stock [Member] | Range Three [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Multiple of Equity Value at issuance	30.0
Per share common stock price required for payment	$ 120
Success payments	$ 70