Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Basic:
Net income	$ 30,547	$ 57,196	$ 81,521	$ 96,988
Net income attributable to participating securities	(742)	(1,192)	(1,982)	(2,022)
Net income attributable to common stock - basic	29,805	56,004	79,539	94,966
Diluted:
Net income	30,547	57,196	81,521	96,988
Net income attributable to participating securities	(742)	(1,191)	(1,980)	(2,021)
Net income attributable to common stock - diluted	$ 29,805	$ 56,005	$ 79,541	$ 94,967
Weighted average shares (denominator):
Weighted average shares - basic	48,303	47,961	48,279	47,930
Diluted effect of stock options	41	45	43	43
Weighted average shares - diluted	48,344	48,006	48,322	47,973
Basic income per common share	$ 0.62	$ 1.17	$ 1.65	$ 1.98
Diluted income per common share	$ 0.62	$ 1.17	$ 1.65	$ 1.98