Reconciliation Between Basic and Diluted Weighted Average Shares Outstanding (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Net income	$ 42,637	$ 49,504	$ 42,471	$ 49,433	$ 36,494	$ 44,173	$ 37,035	$ 46,293	$ 184,045	$ 163,995	$ 161,752
Net income attributable to unvested awarded performance stock									(5,313)	(5,099)	(5,097)
Net income attributable to common shares									$ 178,732	$ 158,896	$ 156,655
Weighted average basic number of common shares outstanding									143,507	143,029	142,412
Less unvested awarded performance stock included in weighted average basic share outstanding									(4,143)	(4,447)	(4,488)
Weighted average number of common shares outstanding for basic earnings per common share									139,364	138,582	137,924
Dilutive effect of stock options									2,646	1,682	1,394
Weighted average number of shares outstanding									142,010	140,264	139,318
Basic	$ 0.30	$ 0.34	$ 0.30	$ 0.34	$ 0.25	$ 0.31	$ 0.26	$ 0.32	$ 1.28	$ 1.15	$ 1.14
Diluted	$ 0.29	$ 0.34	$ 0.29	$ 0.34	$ 0.25	$ 0.30	$ 0.26	$ 0.32	$ 1.26	$ 1.13	$ 1.12