Quarterly Financial Information (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	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, 2012		Dec. 31, 2011		Dec. 31, 2010
Quarterly Financial Data [Abstract]
Revenues	$ 426,434	$ 425,992	$ 384,264	$ 409,343	$ 363,294	$ 340,520	$ 365,240	$ 372,855	$ 1,646,033		$ 1,441,909		$ 1,395,237
Interest expense	8,604	5,912	9,857	9,010	6,416	6,306	6,383	6,242	33,383		25,347		13,211
Net revenues	417,830	420,080	374,407	400,333	356,878	334,214	358,857	366,613	1,612,650		1,416,562		1,382,026
Non-interest expense	356,763	358,403	330,533	341,079	311,852	295,191	354,982	315,929	1,386,778		1,277,954		1,382,255
Income before income tax expense	61,067	61,677	43,874	59,254	45,026	39,023	3,875	50,684	225,872		138,608		(229)
Net income	$ 39,954	$ 37,710	$ 26,136	$ 34,773	$ 27,016	$ 22,304	$ 3,416	$ 31,398	$ 138,573		$ 84,134		$ 1,907
Basic	$ 0.74	$ 0.70	$ 0.49	$ 0.65	$ 0.52	$ 0.43	$ 0.06	$ 0.60	$ 2.59		$ 1.61		$ 0.04
Diluted	$ 0.63	$ 0.60	$ 0.42	$ 0.55	$ 0.43	$ 0.35	$ 0.05	$ 0.50	$ 2.20	[1]	$ 1.33	[1]	$ 0.03	[1]

[1] Diluted earnings per share is computed on the basis of the weighted average number of shares of common stock plus the effect of dilutive potential common shares outstanding during the period using the treasury stock method. Diluted earnings per share include stock options and units.