Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Net income (Numerator)
Net income available to common stockholders - basic	$ 22,312	$ 22,671	$ 44,508	$ 40,132
Net income available to common stockholders - diluted	$ 22,312	$ 22,671	$ 44,508	$ 40,132
Shares (Denominator)
Weighted-Average Shares (Denominator) (shares)	79,490,767	79,062,233	79,481,359	78,746,444
Effect of Dilutive Securities:
Stock options and performance units (shares)	40,119	155,890	49,132	238,957
Common stock warrants (shares)	83,160	18,609	87,955	15,410
Diluted EPS - common stock (shares)	79,614,046	79,236,732	79,618,446	79,000,811
Per Share (Amount)
Basic EPS - common stock (in dollars per share)	$ 0.28	$ 0.29	$ 0.56	$ 0.51
Diluted EPS - common stock (in dollars per share)	$ 0.28	$ 0.29	$ 0.56	$ 0.51
Stock options and restricted shares
Antidilutive Securities Excluded from Computation of Earnings Per Share
Antidilutive shares of common stock	86,103	192,227	80,661	198,565
Warrants
Antidilutive Securities Excluded from Computation of Earnings Per Share
Antidilutive shares of common stock	18,482	18,114	18,482,000	18,114,000