Computation of Basic and Diluted Earnings per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Net Income (Loss) Available to Common Stockholders, Basic
Net income	$ 21,393	$ 20,044
Weighted Average Number of Shares Outstanding, Basic and Diluted
Weighted average common shares outstanding for basic earnings per share computation	43,997,815	43,140,409
Dilutive effect of stock-based compensation	622,961	287,973
Weighted average common shares outstanding for diluted earnings per common share computation	44,620,776	43,428,382
Earnings Per Share, Basic and Diluted
Basic earnings per common share	$ 0.49	$ 0.46
Diluted earnings per common share	$ 0.48	$ 0.46
Stock Options
Earnings Per Share, Basic and Diluted [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount		1,025,419
Unvested Restricted Stock
Earnings Per Share, Basic and Diluted [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	111,417	61,760