Selected Quarterly Results (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended												12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Selected Quarterly Results [Abstract]
Net interest income	$ 20,186	$ 22,948	$ 22,444	$ 22,526	$ 21,145	$ 19,100	$ 19,008	$ 19,147	$ 16,972	$ 14,559	$ 15,033	$ 12,731	$ 88,104	$ 78,401	$ 59,295
Net interest income after provision for loan losses	20,186	22,948	22,444	22,265	21,123	18,990	19,008	19,087	16,851	14,259	14,833	12,481	87,843	78,209	58,424
Net income to common shareholders	19,266	(6,921)	29,442	18,381	18,330	18,353	18,847	16,476	14,406	1,532	13,594	10,280	60,167	72,006	39,812
Comprehensive Income (Loss) to Common Shareholders	$ 19,813	$ (6,184)	$ (67,215)	$ 27,427	$ 14,951	$ 52,737	$ 17,429	$ 42,655	$ 12,756	$ (6,772)	$ 13,910	$ 6,606	$ (26,160)	$ 127,772	$ 26,500
Basic Net Income Per Common Share	$ 0.35	$ (0.13)	$ 0.54	$ 0.34	$ 0.34	$ 0.34	$ 0.35	$ 0.33	$ 0.36	$ 0.04	$ 0.34	$ 0.31	$ 1.10	$ 1.35	$ 1.03
Diluted Net Income Per Common Share	$ 0.35	$ (0.13)	$ 0.54	$ 0.34	$ 0.34	$ 0.34	$ 0.35	$ 0.33	$ 0.36	$ 0.04	$ 0.34	$ 0.31	$ 1.10	$ 1.35	$ 1.03
Dividends declared per common share	$ 0.27	$ 0.27	$ 0.29	$ 0.29	$ 0.29	$ 0.29	$ 0.29	$ 0.28	$ 0.28	$ 0.27	$ 0.27	$ 0.27	$ 1.1200	$ 1.1500	$ 1.0900