Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Basic:
Net income	$ 50,974	$ 39,792
Net income attributable to participating securities	(1,217)	(874)
Net income attributable to common stock - basic	49,757	38,918
Diluted:
Net income	50,974	39,792
Net income attributable to participating securities	(1,216)	(873)
Net income attributable to common stock - diluted	$ 49,758	$ 38,919
Weighted average shares (denominator):
Weighted average shares - basic	48,254	47,898
Diluted effect of stock options	45	41
Weighted average shares - diluted	48,299	47,939
Basic income per common share	$ 1.03	$ 0.81
Diluted income per common share	$ 1.03	$ 0.81