Net Income Per Share - Reconciliation between Basic and Diluted Weighted Average Shares Outstanding (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Earnings Per Share [Abstract]
Net income	$ 67,427	$ 68,331	$ 185,383	$ 182,501
Net income attributable to unvested awarded performance stock	(1,565)	(1,681)	(4,370)	(4,604)
Net income attributable to common shares	$ 65,862	$ 66,650	$ 181,013	$ 177,897
Weighted average number of common shares outstanding - basic (in shares)	140,955	144,469	141,517	144,909
Less unvested awarded performance stock included in weighted average number of common shares outstanding - basic (in shares)	(3,271)	(3,554)	(3,336)	(3,656)
Weighted average number of common shares outstanding for basic earnings per common share (in shares)	137,684	140,915	138,181	141,253
Dilutive effect of stock options (in shares)	2,357	2,049	2,265	1,849
Weighted average number of shares outstanding - diluted (in shares)	140,041	142,964	140,446	143,102
Basic (in dollars per share)	$ 0.48	$ 0.47	$ 1.31	$ 1.26
Diluted (in dollars per share)	$ 0.47	$ 0.47	$ 1.29	$ 1.24