Stockholders' equity - Additional Information (Detail) (USD $)	0 Months Ended	1 Months Ended	5 Months Ended	9 Months Ended		5 Months Ended	9 Months Ended			9 Months Ended			3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended
	May 31, 2014	May 31, 2013	Sep. 30, 2014	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014 Share Repurchase Program Purchase Price [Member]	Sep. 30, 2014 Share Repurchase Program Purchase Price [Member]	Sep. 30, 2014 Restricted Stock [Member]	Dec. 31, 2013 Restricted Stock [Member]	Sep. 30, 2014 Performance Based Restricted Stock Unit [Member]	Sep. 30, 2014 Restricted Stock Units (RSUs) [Member]	Sep. 30, 2014 Performance Based Stock Options [Member]	Sep. 30, 2014 Stock Options [Member]	Sep. 30, 2013 Stock Options [Member]	Sep. 30, 2014 Stock Options [Member]	Sep. 30, 2013 Stock Options [Member]	Sep. 30, 2014 Incentive Stock Options [Member]	Sep. 30, 2013 Incentive Stock Options [Member]	Sep. 30, 2014 Incentive Stock Options [Member]	Sep. 30, 2013 Incentive 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								$ 15.44					$ 15.44		$ 15.44
Unrecognized stock-based compensation cost								$ 11,400,000					$ 32,000		$ 32,000
Weighted average period								30 years
Approximate units of performance restricted stock granted										100,000
Approximate units outstanding								1,268,000	1,279,000	300,000
Performance criteria met for approximate units								369,000		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				1,262,000	221,000						1,300,000
Aggregate intrinsic value, options exercised													1,700,000	5,300,000	14,400,000	9,100,000
Weighted average vesting period				9 months
Performance-based stock options Cumulative Granted												400,000
Performance options, vested												300,000
Number of options awarded				0	0
Incremental stock option expense																	2,000	22,000	18,000	100,000
Stock repurchase authorized amount	30,000,000
Purchased common shares under stock repurchase program			1,700,000	200,000
Purchased common shares under stock repurchase program, total cost			22,400,000	2,900,000
Purchased common shares under stock repurchase program, average cost per share			$ 12.99	$ 11.94		$ 12.96	$ 11.91
Remaining stock repurchase authorized amount							$ 7,600,000