Stockholders' equity - Additional Information (Detail) (USD $)	0 Months Ended		3 Months Ended	6 Months Ended		3 Months Ended	6 Months Ended	3 Months Ended		6 Months Ended				6 Months Ended	3 Months Ended		6 Months Ended
	May 31, 2014	May 23, 2013	Jun. 30, 2014	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014 Share Repurchase Program Purchase Price [Member]	Jun. 30, 2014 Restricted Stock Units (RSUs) [Member]	Jun. 30, 2014 Incentive Stock Options [Member]	Jun. 30, 2013 Incentive Stock Options [Member]	Jun. 30, 2014 Incentive Stock Options [Member]	Jun. 30, 2013 Incentive Stock Options [Member]	Jun. 30, 2014 Restricted Stock [Member]	Dec. 31, 2013 Restricted Stock [Member]	Jun. 30, 2014 Performance Based Restricted Stock Unit [Member]	Jun. 30, 2014 Stock Options [Member]	Jun. 30, 2013 Stock Options [Member]	Jun. 30, 2014 Stock Options [Member]	Jun. 30, 2014 Performance Based Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock incentive plan		1,600,000
Option available for future grants			1,900,000	1,900,000
Aggregate intrinsic value price per share												$ 13.30			$ 13.30		$ 13.30
Unrecognized stock-based compensation cost												$ 12,300,000			$ 49,000		$ 49,000
Weighted average period												30 months
Approximate units of performance restricted stock granted														100,000
Approximate units outstanding												1,258,773	1,279,123	300,000
Performance criteria met for approximate units												348,171		100,000
Maximum additional shares														100,000
Shares withheld, for net share settlement of share based award							100,000
Tax payments, for net share settlement of share based award				1,190,000	185,000		1,200,000
Aggregate intrinsic value, options exercised															200,000	2,300,000
Weighted average vesting period				10 months
Performance-based stock options Cumulative Granted																		400,000
Performance options, vested																		300,000
Number of options awarded				0	0
Incremental stock option expense								8,000	24,000	17,000	63,000
Stock repurchase authorized amount	30,000,000					10,400,000
Purchased common shares under stock repurchase program			1,500,000
Purchased common shares under stock repurchase program, total cost			$ 19,600,000
Purchased common shares under stock repurchase program, average cost per share			$ 13.16			$ 13.13