Computation of Basic and Diluted Earnings per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Net Income (Loss) Available to Common Stockholders, Basic
Net income	$ 20,124	$ 20,980
Weighted Average Number of Shares Outstanding, Basic and Diluted
Weighted average common shares outstanding for basic earnings per share computation	44,719,218	45,378,230
Dilutive effect of stock-based compensation	394,834	461,961
Weighted average common shares outstanding for diluted earnings per common share computation	45,114,052	45,840,191
Earnings Per Share, Basic and Diluted
Basic earnings per common share	$ 0.45	$ 0.46
Diluted earnings per common share	$ 0.45	$ 0.46
Performance Shares [Member]
Earnings Per Share, Basic and Diluted [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	226,821	166,770
Stock Options
Earnings Per Share, Basic and Diluted [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	0	5,000
Unvested Restricted Stock
Earnings Per Share, Basic and Diluted [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	40,865