Earnings per share - Additional Information (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Warrant exercise price		12.74
Earnings per share, dilutive effect, description	Because the Company would have exercised a call option if the market price of our stock exceeded the warrant exercise price of $12.74 per share, the effect of the convertible note hedge, which expired pro rata as the 2014 Notes were repurchased, was excluded from the calculation of diluted earnings per share for the three months ended March 31, 2014, as the impact was always considered anti-dilutive.
7.75% Convertible notes due 2018
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Debt instrument, interest rate percentage	7.75%	7.75%	7.75%
Common shares excluded from the computation of diluted net income (loss) per share	4,832	4,832
4.5% Convertible notes due 2014
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Debt instrument, interest rate percentage		4.50%
Stock Options And Restricted Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Number of shares excluded from computation of diluted net income per share	9,500