Stockholders' Equity (Detail) $ / shares in Units, $ in Millions	May. 29, 2015 USD ($) agreement $ / shares shares	Sep. 30, 2015 $ / shares shares	Sep. 08, 2015 $ / shares shares	Sep. 07, 2015 shares	Dec. 31, 2014 $ / shares shares
Stockholders Equity [Line Items]
Number of Investment Agreements entered into | agreement	15
Common stock, shares issued | shares		108,373,631			77,421,683
Common stock, par value (in usd per share)		$ 0.001	$ 0.001		$ 0.001
Series A Preferred stock, par value (in usd per share)		$ 0.001			$ 0.001
Allocated intrinsic value | $	$ 52.0
Common stock, shares authorized | shares		300,000,000	300,000,000	150,000,000	150,000,000
Purchased Common Shares
Stockholders Equity [Line Items]
Common stock, shares issued | shares	15,499,445
Common stock, par value (in usd per share)	$ 0.001
Shares issued, price per share (in usd per share)	$ 45.00
Proceeds from issuance of private placement | $	$ 697.5
Common stock, price per share (in usd per share)	$ 49.16
Series C Preferred Stock
Stockholders Equity [Line Items]
Series A Preferred stock, shares issued | shares	562,525
Series A Preferred stock, par value (in usd per share)	$ 0.001
Shares issued, price per share (in usd per share)	$ 1,000
Proceeds from issuance of private placement | $	$ 562.5
Net proceeds from issuance of Purchased Securities, net of issuance costs | $	$ 1,228.1
Aggregate common shares issued upon conversion | shares	12,500,546
Conversion price (in usd per share)	$ 45.00