Share-based Compensation plans - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Jun. 29, 2004
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Options outstanding	1,412,602	1,527,836
Options exercisable	795,350
Purchase price of shares equivalent to fair market value	85.00%
Unamortized compensation expense	$ 2,600
Weighted average remaining contractual life of exercisable options (in years)	2 years 11 months 16 days
Total intrinsic value of options exercised	$ 700	$ 2,100	$ 400
Closing stock price	$ 39.21
Aggregate intrinsic value of options outstanding	$ 8,000	2,400	500
Aggregate intrinsic value of options exercisable	4,300	1,000	200
Share-based compensation	7,214	$ 5,724	6,267
Restricted Stock [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Unamortized compensation expense	$ 16,100
Non-vested shares	314,278	358,450
Non-vested shares, vested in period	$ 5,600	$ 2,500	2,100
Weighted-average period for unamortized compensation cost expected to be recognized	2 years 4 months 24 days
Aggregate intrinsic value of restricted stock outstanding	$ 22,400	$ 13,900	6,500
Performance Based [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Restricted stock outstanding	192,310	86,050
Unamortized compensation expense	$ 6,600
Non-vested shares	110,660
Share-based compensation	$ 0	$ 0	$ 0
2012 LTIP Plan [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Amount of shares reserved for issuance	3,200,000
Options outstanding	653,040
Options exercisable	185,788
Restricted stock outstanding	571,488
2004 LTIP Plan [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Amount of shares reserved for issuance				3,100,000
Full value award limit	100,000
2004 LTIP Plan [Member] | Staff Share Option Plan [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Options outstanding	609,562
Options exercisable	609,562
Restricted stock outstanding	0
Award Contractual term	10 years
2004 LTIP Plan [Member] | Employees [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Award vesting period	3 years
2004 LTIP Plan [Member] | Non-Employees Director [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Number of shares approved for annual grant	5,000
2004 LTIP Plan [Member] | Minimum [Member] | Non-Employees Director [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Award vesting period	3 years
2004 LTIP Plan [Member] | Maximum [Member] | Non-Employees Director [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Award vesting period	5 years
Stock Purchase Plan [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Amount of shares reserved for issuance	1,850,000
Maximum percentage of compensation eligible employees to be deducted for purchase of common stock	25.00%
Shares issued under stock purchase plan	1,617,827
Stock Purchase Plan [Member] | Minimum [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Percentage of compensation eligible employees to be deducted for purchase of common stock	1.00%