Convertible Senior Notes - Additional Information (Detail)			1 Months Ended						2 Months Ended	3 Months Ended			12 Months Ended
	Nov. 01, 2018 $ / shares	Jul. 26, 2018 USD ($)	Nov. 30, 2017 USD ($)	Aug. 31, 2017 USD ($)	Jun. 30, 2017 USD ($) shares	Jan. 31, 2016 USD ($)	Jun. 30, 2014 USD ($) $ / shares	Jul. 31, 2013 USD ($) $ / shares	Feb. 28, 2017 USD ($) shares	Dec. 31, 2017 USD ($)	Jun. 30, 2017 USD ($) shares	Mar. 31, 2017 USD ($) shares	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($)	Aug. 01, 2018 USD ($)
Debt Instrument [Line Items]
Long term debt, face amount										$ 156,647,000			$ 140,974,000	$ 156,647,000
Gain (loss) on extinguishment of convertible senior notes													(453,000)	(611,000)	$ 98,000
Payment for repurchase of convertible notes													0	35,614,000	8,035,000
Change in fair value of convertible senior notes													2,948,000	(308,000)	0
Convertible senior notes payable, fair value										154,572,000			139,792,000	154,572,000
4.25% Convertible Senior Notes (due 2018)
Debt Instrument [Line Items]
Long term debt, face amount								$ 100,000,000.0		$ 21,178,000			$ 0	$ 21,178,000
Debt instrument, interest rate		4.25%		4.25%				4.25%		4.25%			4.25%	4.25%
Debt conversion ratio								0.1143674
Debt instrument, conversion price (USD per share) | $ / shares								$ 8.74
Debt instrument repurchase amount					$ 12,000,000						$ 12,000,000	$ 39,100,000			6,100,000
Write-off of debt issuance costs											100,000				$ 100,000
Gain (loss) on extinguishment of convertible senior notes											100,000
Payment for repurchase of convertible notes					11,600,000							$ 24,100,000
Debt instrument shares common stock issued upon conversion (in shares) | shares												2,900,000
Convertible senior notes payable, fair value										$ 21,075,000			$ 0	$ 21,075,000
4.25% Convertible Senior Notes (due 2018) | Common Stock
Debt Instrument [Line Items]
Debt instrument repurchase amount					$ 11,600,000				$ 39,100,000		$ 11,600,000
Debt instrument shares common stock issued upon conversion (in shares) | shares					112,400				2,865,000		112,400			3,000,000
3.25% Convertible senior notes due in 2020
Debt Instrument [Line Items]
Long term debt, face amount										22,469,000			$ 27,974,000	$ 22,469,000
Debt instrument, interest rate		3.25%											3.25%
Convertible senior notes payable, fair value										22,469,000			$ 27,974,000	22,469,000
4.875% convertible senior notes (due 2020)
Debt Instrument [Line Items]
Long term debt, face amount							$ 115,000,000			113,000,000			$ 113,000,000	113,000,000
Debt instrument, interest rate							4.875%						4.875%
Debt conversion ratio							0.1037613
Debt instrument, conversion price (USD per share) | $ / shares							$ 9.64
Debt instrument repurchase amount						$ 2,000,000
Write-off of debt issuance costs						100,000
Gain (loss) on extinguishment of convertible senior notes						$ 100,000
Convertible senior notes payable, fair value										111,028,000			$ 111,818,000	111,028,000
Convertible senior note payable, fair value										89,700,000			93,200,000	89,700,000
Oasis Management and Oasis Investments ll Master Fund Ltd. | 4.25% Convertible Senior Notes (due 2018)
Debt Instrument [Line Items]
Debt instrument, interest rate				4.25%
Oasis Management and Oasis Investments ll Master Fund Ltd. | 3.25% Convertible senior notes due in 2020
Debt Instrument [Line Items]
Long term debt, face amount		$ 8,000,000		$ 21,500,000						21,500,000			$ 29,500,000	$ 21,500,000
Debt conversion ratio	0.3937008	0.3222688		0.3280302
Debt instrument, conversion price (USD per share) | $ / shares	$ 2.54
Gain (loss) on extinguishment of convertible senior notes			$ (500,000)							$ (600,000)
Debt prepayment percentage	105.00%
Debt instrument, amount redeemed																$ 13,200,000