Earnings Per Share - Summary of Earnings Per Share, Basic and Diluted (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Earnings Per Share [Abstract]
Net income	$ 109,540	$ 95,509	$ 88,549	$ 75,109	$ 68,120	$ 78,620	$ 73,099	$ 64,662	$ 368,707	$ 284,501	$ 216,199
Denominator for basic earnings per share	87,877	83,611	82,996	82,737	82,388	81,974	81,573	81,222	84,317	81,793	83,328
Dilutive securities									2,665	2,862	2,408
Denominator for diluted earnings per share	90,240	86,134	85,817	85,695	85,277	84,905	84,461	83,960	86,982	84,655	85,736
Basic earnings per share	$ 1.25	$ 1.14	$ 1.07	$ 0.91	$ 0.83	$ 0.96	$ 0.90	$ 0.80	$ 4.37	$ 3.48	$ 2.59
Diluted earnings per share	$ 1.21	$ 1.11	$ 1.03	$ 0.88	$ 0.80	$ 0.93	$ 0.87	$ 0.77	$ 4.24	$ 3.36	$ 2.52