Summary Of Significant Accounting Policies (Narrative) (Detail) (USD $)	1 Months Ended	3 Months Ended	12 Months Ended		12 Months Ended							12 Months Ended
	Sep. 21, 2012	Dec. 31, 2012	Dec. 31, 2012 Y Segment	Dec. 31, 2011	Dec. 31, 2012 Astellas [Member] Collaborative Agreement [Member]	Dec. 31, 2012 Pfizer [Member] Collaborative Agreement [Member]	Dec. 31, 2012 Cash And Cash Equivalents [Member]	Dec. 31, 2012 Short-Term Investments [Member]	Dec. 31, 2012 Capital Stock [Member]	Sep. 20, 2012 Capital Stock [Member]	Dec. 31, 2011 Capital Stock [Member]	Dec. 31, 2012 Common Stock [Member] Ratio	Sep. 20, 2012 Common Stock [Member]	Dec. 31, 2011 Common Stock [Member]
Number of operating business segment			1
Shares of capital stock authorized pre-split										86,000,000	51,000,000
Shares of common stock authorized pre-split													85,000,000	50,000,000
Shares of capital stock authorized									171,000,000
Total number of authorized shares		170,000,000	170,000,000	170,000,000								170,000,000
Forward stock split of common stock												2
Common stock issued in forward stock split			37,000,000									37,000,000
Par value of common stock		$ 0.01	$ 0.01	$ 0.01
Debt instrument convertible increment of principal amount of conversion	$ 1,000		$ 1,000
Conversion rate of common stock, shares per principal amount		19.5172
Conversion price, per share of common stock		$ 51.24	$ 51.24
Highly liquid investments maximum maturity period							3 months	12 months
Contract term of the Convertible Notes, years			5
Non-refundable up-front payments					$ 110,000,000	$ 225,000,000