Net Income per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Numerator
Net income	$ 34,591	$ 16,917	$ 58,010	$ 24,800
Denominator
Weighted-average shares outstanding, basic (in shares)	72,379,000	41,493,000	72,336,000	33,962,000
Effect of dilutive nonvested stock awards (in shares)	205,000	276,000	206,000	308,000
Adjusted weighted-average shares outstanding, diluted (in shares)	72,584,000	41,769,000	72,542,000	34,270,000
Net Income per Share
Basic earnings per common share (in dollars per share)	$ 0.48	$ 0.41	$ 0.80	$ 0.73
Diluted earnings per common share (in dollars per share)	$ 0.48	$ 0.41	$ 0.80	$ 0.72
Antidilutive awards excluded from computation of earnings per common share (in shares)	40,000		26,000	0