Quarterly Financial Information (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 590,636	$ 534,683	$ 568,989	$ 555,377	$ 574,156	$ 490,174	$ 506,312	$ 449,173	$ 2,249,685		$ 2,019,814		$ 1,627,483
Interest expense	12,560	11,228	8,842	8,631	11,630	11,535	12,634	10,569	41,261		46,368		33,370
Net revenues	578,076	523,455	560,147	546,746	562,526	478,639	493,678	438,604	2,208,424	[1]	1,973,446	[1]	1,594,113	[1]
Total non-interest expense	508,698	457,689	482,624	468,618	484,537	447,631	441,103	414,946	1,917,630		1,788,217		1,364,366
Income from continuing operations before income taxes	69,378	65,766	77,523	78,128	77,989	31,008	52,575	23,658	290,794		185,229		229,747
Provision for income taxes	23,890	25,673	31,946	30,155	25,863	(43,921)	21,659	8,722	111,664		12,322		84,451
Income from continuing operations	45,488	40,093	45,577	47,973	52,126	74,929	30,916	14,936	179,130		172,907		145,296
Income/(loss) from discontinued operations	(306)	(190)	(1,976)	(591)	(3,857)	(5,239)	(1,481)	(317)	(3,063)		(10,894)		(6,723)
Net income	$ 45,182	$ 39,903	$ 43,601	$ 47,382	$ 48,269	$ 69,690	$ 29,435	$ 14,619	$ 176,067		$ 162,013		$ 138,573
Earnings per basic common share
Income from continuing operations	$ 0.67	$ 0.60	$ 0.69	$ 0.73	$ 0.80	$ 1.16	$ 0.48	$ 0.25	$ 2.69		$ 2.72		$ 2.71
Loss from discontinued operations			$ (0.03)	$ (0.01)	$ (0.06)	$ (0.08)	$ (0.02)	$ (0.01)	$ (0.04)		$ (0.17)		$ (0.12)
Earnings per basic common share	$ 0.67	$ 0.60	$ 0.66	$ 0.72	$ 0.74	$ 1.08	$ 0.46	$ 0.24	$ 2.65		$ 2.55		$ 2.59
Earnings per diluted common share
Income from continuing operations	$ 0.59	$ 0.52	$ 0.60	$ 0.63	$ 0.69	$ 1.00	$ 0.42	$ 0.22	$ 2.35		$ 2.35		$ 2.31
Loss from discontinued operations	$ (0.01)		$ (0.02)		$ (0.05)	$ (0.07)	$ (0.02)	$ (0.01)	$ (0.04)		$ (0.15)		$ (0.11)
Earnings per diluted common share	$ 0.58	$ 0.52	$ 0.58	$ 0.63	$ 0.64	$ 0.93	$ 0.40	$ 0.21	$ 2.31		$ 2.20		$ 2.20
Weighted average number of common shares outstanding:
Basic	66,851	66,691	66,302	66,037	64,859	64,706	64,505	60,054	66,472		63,568		53,563
Diluted	77,540	76,681	75,641	75,691	75,495	75,191	74,090	69,189	76,376		73,504		62,937

[1] No individual client accounted for more than 10 percent of total net revenues for the years ended December 31, 2014, 2013, or 2012.