Quarterly Financial Information (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 666,791	$ 617,050	$ 625,590	$ 639,077	$ 697,335	$ 620,932	$ 633,340	$ 590,474
Interest income	184,534	169,760	154,421	137,734	126,615	117,862	108,951	100,953	$ 646,449		$ 454,381		$ 294,332
Total revenues	851,325	786,810	780,011	776,811	823,950	738,794	742,291	691,427	3,194,957		2,996,462		2,642,370
Interest expense	57,876	48,468	37,279	26,453	19,865	17,625	16,644	15,896	170,076		70,030		66,874
Net revenues	793,449	738,342	742,732	750,358	804,085	721,169	725,647	675,531	3,024,881	[1]	2,926,432	[1]	2,575,496	[1]
Total non-interest expenses	637,518	597,812	624,385	630,804	804,905	613,030	642,449	596,512	2,490,519		2,656,896		2,432,914
Income before income tax expense	155,931	140,530	118,347	119,554	(820)	108,139	83,198	79,019	534,362		269,536		142,582
Provision for income taxes	41,869	36,672	31,060	30,793	1,168	41,603	30,387	13,507	140,394		86,665		61,062
Net income	114,062	103,858	87,287	88,761	(1,988)	66,536	52,811	65,512	393,968		182,871		81,520
Preferred dividends	2,344	2,343	2,344	2,344	2,344	2,343	2,344	2,344	9,375		9,375		3,906
Net Income available to common shareholders	$ 111,718	$ 101,515	$ 84,943	$ 86,417	$ (4,332)	$ 64,193	$ 50,467	$ 63,168	$ 384,593		$ 173,496		$ 77,614
Earnings per common share
Basic	$ 1.56	$ 1.41	$ 1.18	$ 1.20	$ (0.06)	$ 0.94	$ 0.74	$ 0.92	$ 5.36		$ 2.53		$ 1.16
Diluted	$ 1.38	$ 1.25	$ 1.04	$ 1.06	$ (0.06)	$ 0.79	$ 0.63	$ 0.78	$ 4.73		$ 2.14		$ 1.00
Weighted-average number of common shares outstanding:
Basic	71,666	71,919	71,692	71,999	68,782	68,522	68,556	68,386	71,786		68,562		66,871
Diluted	80,706	81,484	81,299	81,789	68,782	80,881	80,021	80,695	81,321		81,035		77,563
Cash dividends declared per common share	$ 0.12	$ 0.12	$ 0.12	$ 0.12	$ 0.10	$ 0.10			$ 0.48		$ 0.20		$ 0

[1] No individual client accounted for more than 10 percent of total net revenues for the years ended December 31, 2018, 2017, and 2016.