Computation of Basic and Diluted 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 (Loss) Available to Common Stockholders, Basic
Net income	$ 21,077	$ 21,505	$ 42,470	$ 41,549
Weighted Average Number of Shares Outstanding, Basic and Diluted
Weighted average common shares outstanding for basic earnings per share computation	44,044,260	43,480,502	44,021,166	43,335,682
Dilutive effect of stock-based compensation	531,703	427,785	572,689	376,978
Weighted average common shares outstanding for diluted earnings per common share computation	44,575,963	43,908,287	44,593,855	43,712,660
Earnings Per Share, Basic and Diluted
Basic earnings per common share	$ 0.48	$ 0.49	$ 0.96	$ 0.96
Diluted earnings per common share	$ 0.47	$ 0.49	$ 0.95	$ 0.95
Stock Options
Earnings Per Share, Basic and Diluted [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount			0	976,063
Unvested Restricted Stock
Earnings Per Share, Basic and Diluted [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount			107,651	56,668