Earnings Per Common Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Earnings Per Common Share
Net income	$ 13,852	$ 9,996
Income allocated to participating securities	(158)
Income attributable to common shareholders	$ 13,694	$ 9,996
Average number of common shares outstanding - basic	96,674,729	95,018,867
Average number of common shares outstanding - diluted	96,674,729	95,018,867
Income per common share attributable to common shareholders:
Basic (in dollars per share)	$ 0.14	$ 0.11
Diluted (in dollars per share)	$ 0.14	$ 0.11
Stock options excluded from computation of earnings per share (in shares)	2,825,855	5,822,224