Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Earnings Per Share
Net income	$ 53,091	$ 67,209	$ 73,734
Income allocated to participating securities (restricted stock)	(463)	(1,685)	(1,752)
Income attributable to common shareholders	$ 52,628	$ 65,524	$ 71,982
Average number of common shares outstanding - basic	95,455,344	93,253,928	91,776,907
Dilutive effect of stock options (in shares)			34
Average number of common shares outstanding - diluted	95,455,344	93,253,928	91,776,941
Income per common share attributable to common shareholders:
Basic (in dollars per share)	$ 0.55	$ 0.70	$ 0.78
Diluted (in dollars per share)	$ 0.55	$ 0.70	$ 0.78
Stock options excluded from computation of earnings per share (in shares)	5,495,748	6,846,339	7,934,284