Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Net income	$ 70,625	$ 78,982	$ 77,543	$ 57,223	$ 45,639	$ 43,232	$ 36,812	$ 43,734	$ 284,373	$ 169,500	$ 174,400
Weighted average shares	76,606,611	75,743,002	74,807,174	59,442,488	46,100,493	38,765,078	36,609,290	36,241,823	71,708,058	39,454,362	36,195,840
Basic earnings per share	$ 0.92	$ 1.04	$ 1.04	$ 0.96	$ 0.99	$ 1.12	$ 1.01	$ 1.21
Diluted earnings per share	$ 0.92	$ 1.04	$ 1.04	$ 0.96	$ 0.99	$ 1.11	$ 1.01	$ 1.21
Earnings Per Share, Basic [Member]
Net income									284,373	169,500	174,400
Weighted average shares									71,708,058	39,454,362	36,196,840
Basic earnings per share									$ 3.97	$ 4.30	$ 4.82
Earnings Per Share, Diluted [Member]
Net income									$ 284,373	$ 169,500	$ 174,400
Weighted average shares									71,708,058	39,454,362	36,196,840
Potential dilutive shares from exercise of stock options											19,525
Diluted weighted average shares									71,708,058	39,454,362	36,216,365
Diluted earnings per share									$ 3.97	$ 4.30	$ 4.82