Calculation of Basic and Diluted Per Share Of Common Stock (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended																								12 Months Ended
	Dec. 31, 2017	[1]	Sep. 30, 2017	[1]	Jun. 30, 2017	[1]	Mar. 31, 2017	[1]	Dec. 31, 2016	[1]	Sep. 30, 2016	[1]	Jun. 30, 2016	[1]	Mar. 31, 2016	[1]	Dec. 31, 2015	[1]	Sep. 30, 2015	[1]	Jun. 30, 2015	[1]	Mar. 31, 2015	[1]	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Basic net income per share:
Net income applicable to common stockholders																									$ 111,422	$ 119,433	$ 104,289
Weighted average common shares outstanding																									59,358	63,729	67,201
Basic net income per share	$ 0.84		$ 0.32		$ 0.38		$ 0.34		$ 0.37		$ 0.52		$ 0.55		$ 0.42		$ 0.50		$ 0.48		$ 0.42		$ 0.15		$ 1.88	$ 1.87	$ 1.55
Diluted net income per share:
Net income applicable to common stockholders																									$ 111,422	$ 119,433	$ 104,289
Weighted average common shares outstanding																									59,358	63,729	67,201
Dilutive shares from assumed conversion of convertible senior notes																									1,726	794	1,128
Dilutive stock options and restricted stock																									1,515	1,239	1,039
Diluted weighted average common shares outstanding																									62,599	65,762	69,368
Diluted net income per share	$ 0.80		$ 0.30		$ 0.36		$ 0.32		$ 0.36		$ 0.51		$ 0.53		$ 0.42		$ 0.50		$ 0.47		$ 0.41		$ 0.15		$ 1.78	$ 1.82	$ 1.5

[1] Basic and diluted net income per share is computed independently for each of the quarters presented. Therefore, the sum of the quarterly net income per share may differ from annual net income per share due to rounding.