Computation of Basic and Diluted Earnings per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Net Income (Loss) Available to Common Stockholders, Basic
Net income	$ 19,155	$ 23,812	$ 61,625	$ 65,361
Weighted Average Number of Shares Outstanding, Basic and Diluted
Weighted average common shares outstanding for basic earnings per share computation	44,911,858	43,699,566	44,321,326	43,458,309
Dilutive effect of stock-based compensation	548,430	585,278	562,667	384,980
Weighted average common shares outstanding for diluted earnings per common share computation	45,460,288	44,284,844	44,883,993	43,843,289
Earnings Per Share, Basic and Diluted
Basic earnings per common share	$ 0.43	$ 0.54	$ 1.39	$ 1.50
Diluted earnings per common share	$ 0.42	$ 0.54	$ 1.37	$ 1.49
Stock Options
Earnings Per Share, Basic and Diluted [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount			0	875,714
Unvested Restricted Stock
Earnings Per Share, Basic and Diluted [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount			102,529	54,402