Convertible Notes (Narrative) (Detail) $ / shares in Units, shares in Millions						9 Months Ended
	Feb. 02, 2018 USD ($)	Nov. 21, 2016 USD ($)	Nov. 20, 2015 USD ($)	Feb. 11, 2014 USD ($)	Feb. 05, 2014 USD ($)	Sep. 30, 2018 USD ($) $ / shares shares	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Nov. 22, 2016 USD ($) $ / shares	Feb. 12, 2014 USD ($)	Feb. 06, 2014 USD ($) $ / shares
Debt Instrument [Line Items]
Principal amount a $5 incentive cash payment per each to convert debt						$ 1,000
Convertible note rate conversion trading days (in days)						0
Convertible Notes rate conversion consecutive trading days (in days)						$ 30
Minimum conversion price percent for note conversion (in Percent)						130.00%
Interest Paid						$ 4,591,000	$ 7,224,000
Gain on conversion of convertible notes							$ (12,200,000)
Share Price | $ / shares											$ 7.97
Repayments of Debt	$ 129,000,000
Debt Instrument, Periodic Payment, Principal	126,400,000
Debt Instrument, Periodic Payment, Interest	$ 2,600,000
February 2018 Notes [Member]
Debt Instrument [Line Items]
Convertible notes						0		$ 126,066,000		$ 300,000,000
Debt instrument, interest rate (in Percent)										4.00%
Unamortized discount of liability component			$ (3,100,000)			0		(381,000)	$ (4,300,000)	$ (29,700,000)
Deferred issuance costs			900,000						1,300,000
Purchase call option unwind			270,000
Warrant unwind			170,000
Fees and Commissions, Transfer Agent			100,000
Debt discount recorded to additional paid in capital										19,300,000
Debt discount recorded to deferred tax liability										$ 10,400,000
Debt Instrument, Interest Rate, Effective Percentage										6.90%
Estimated market interest rate for similar nonconvertible instrument										7.00%
Net proceeds from the issuance of convertible notes				$ 290,200,000
Debt Instrument, Repurchase Amount			53,600,000						120,000,000
Debt instrument, repurchase amount paid			43,700,000						121,500,000
Interest Paid		$ 1,500,000
Debt Instrument, Face Amount						$ 0		126,447,000
Gain on conversion of convertible notes			(6,500,000)
February 2018 Note Warrant [Member]
Debt Instrument [Line Items]
Convertible notes, conversion price (in Dollars per Share) | $ / shares						$ 10.3610
Proceeds from Issuance of Warrants					$ 11,400,000
December 2021 Notes [Member]
Debt Instrument [Line Items]
Convertible notes			$ 150,000,000			$ 122,780,000		117,415,000	$ 150,000,000
Debt instrument, interest rate (in Percent)									2.80%
Unamortized discount of liability component						$ (27,220,000)		(32,585,000)	$ (4,300,000)
Debt discount recorded to additional paid in capital									23,800,000
Debt discount recorded to deferred tax liability									$ 12,800,000
Maximum percent of common stock closing price and conversion rate to convert note (in Percent)						98.00%
Debt Instrument, Interest Rate, Effective Percentage									3.40%
Convertible notes, conversion price (in Dollars per Share) | $ / shares									$ 3.81
Estimated market interest rate for similar nonconvertible instrument									9.50%
Debt instrument, convertible, remaining amortization period (in Duration)						3 years 2 months 3 days
Net proceeds from the issuance of convertible notes		$ 145,700,000
Debt Instrument, Face Amount						$ 150,000,000		$ 150,000,000
Conversion Rate per $1,000 Principal Amount (in Ratio)		262.2951
Proceeds from Issuance of Warrants		$ 14,400,000
February 2018 Note Purchase Call Option [Member]
Debt Instrument [Line Items]
Purchased call options cost				31,000,000
Number of shares of common stock covered by the purchased call options purchased (in Shares) | shares						13.8
Deferred taxes included in purchased call options cost				$ 10,800,000
Purchased Call Options [Member]
Debt Instrument [Line Items]
Convertible notes, conversion price (in Dollars per Share) | $ / shares						$ 4.88
Series 2012 Notes [Member]
Debt Instrument [Line Items]
Convertible notes											$ 131,700,000