Earnings Per Share - Reconciliation of Numerators and Denominators Used in Computations of Both Basic and Diluted Net Income Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Basic earnings per share computation:
Net income	$ 37,444	$ 34,407	$ 70,791	$ 66,767
Weighted average common shares - basic (in shares)	175,317,750	173,966,917	175,007,291	173,796,646
Basic earnings per share (in dollars per share)	$ 0.21	$ 0.20	$ 0.40	$ 0.38
Diluted earnings per share computation:
Net income	$ 37,444	$ 34,407	$ 70,791	$ 66,767
Weighted average common shares - basic (in shares)	175,317,750	173,966,917	175,007,291	173,796,646
Weighted average common shares - diluted (in shares)	180,044,401	177,589,222	179,662,304	177,200,630
Diluted earnings per share (in dollars per share)	$ 0.21	$ 0.19	$ 0.39	$ 0.38
Equity Option [Member]
Diluted earnings per share computation:
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount ( in shares)		8,705		39,366
Restricted Stock Units (RSUs) [Member]
Diluted earnings per share computation:
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount ( in shares)		6,724	261	4,714
Employee Stock Option [Member]
Diluted earnings per share computation:
Stock option conversion and Stock units (in shares)	4,236,404	3,090,807	4,095,672	2,839,542
Stock Units [Member]
Diluted earnings per share computation:
Stock option conversion and Stock units (in shares)	490,247	531,498	559,341	564,442