Calculation of Net Income Per Share (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Net income	$ 66,358	$ 57,660	$ 185,310	$ 151,323
Income allocated to participating securities-basic		278	3	776
Net income available to common stockholders-basic	66,358	57,382	185,307	150,547
Basic weighted average shares	135,727	137,918	136,469	138,627
Basic net income per share	$ 0.49	$ 0.42	$ 1.36	$ 1.09
Net income	66,358	57,660	185,310	151,323
Income allocated to participating securities-diluted		277	3	772
Net income available to common stockholders-diluted	$ 66,358	$ 57,383	$ 185,307	$ 150,551
Basic weighted average shares	135,727	137,918	136,469	138,627
Dilutive effect of potential common shares	1,514	1,223	1,290	1,108
Diluted weighted average shares	137,241	139,141	137,759	139,735
Diluted net income per share	$ 0.48	$ 0.41	$ 1.35	$ 1.08