Quarterly Financial Data (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended															12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Dec. 31, 2010		Sep. 30, 2010		Jun. 30, 2010		Mar. 31, 2010		Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Total interest income	$ 132,248	$ 133,555		$ 133,217		$ 139,235		$ 146,365		$ 145,337		$ 138,814		$ 111,854		$ 538,256	$ 542,370	$ 459,527
Total interest expense	11,282	14,163		15,774		17,411		18,501		21,844		22,047		21,460		58,629	83,851	110,763
Net interest income	120,966	119,392		117,443		121,824		127,864		123,493		116,767		90,394		479,627	458,519	348,764
Provision for noncovered loan losses	12,275	14,604		10,138		17,018		19,816		18,108		20,366		25,493		54,035	83,783	98,433
Provision for covered loan losses	2,773	4,768		7,481		5,331		3,572		593		267		0		20,353	4,432	0
Net income	30,496	31,737	[1]	29,763	[1]	27,560	[1]	27,030	[1]	28,996	[1]	31,493	[1]	15,390	[1]	119,558	102,909	82,170
Earnings Per Share, Basic	$ 0.28	$ 0.29		$ 0.27		$ 0.25		$ 0.25		$ 0.27		$ 0.32		$ 0.18		$ 1.10	$ 1.02	$ 0.90
Earnings Per Share, Diluted	$ 0.28	$ 0.29		$ 0.27		$ 0.25		$ 0.25		$ 0.27		$ 0.32		$ 0.18		$ 1.10	$ 1.02	$ 0.90
George Washington [Member]
Business Combination, Bargain Purchase, Gain Recognized, Amount														$ 1,000

[1] * Net income restated to reflect purchase accounting adjustments which decreased the bargain purchase gain recognized on the George Washington acquisition to $1.0 million in the quarter ended March 31, 2010.