Calculation of Net Income Per Share (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Earnings Per Share [Abstract]
Net income	$ 66,885	$ 66,358	$ 63,089	$ 55,863	$ 58,619	$ 57,660	$ 45,329	$ 48,334	$ 252,195	$ 209,942	$ 149,922
Income allocated to participating securities-basic									3	1,081	2,159
Net income available to common stockholders-basic									252,192	208,861	147,763
Basic weighted average shares									136,153	138,201	140,479
Basic net income per share	$ 0.49	$ 0.49	$ 0.46	$ 0.41	$ 0.43	$ 0.42	$ 0.33	$ 0.34	$ 1.85	$ 1.51	$ 1.05
Net income	66,885	66,358	63,089	55,863	58,619	57,660	45,329	48,334	252,195	209,942	149,922
Income allocated to participating securities-diluted									3	1,075	2,150
Net income available to common stockholders-diluted	$ 66,885	$ 66,358	$ 63,088	$ 55,861	$ 58,322	$ 57,383	$ 45,101	$ 48,070	$ 252,192	$ 208,867	$ 147,772
Basic weighted average shares									136,153	138,201	140,479
Dilutive effect of potential common shares									1,436	1,208	1,311
Diluted weighted average shares									137,589	139,409	141,790
Diluted net income per share	$ 0.49	$ 0.48	$ 0.46	$ 0.40	$ 0.42	$ 0.41	$ 0.32	$ 0.34	$ 1.83	$ 1.50	$ 1.04