Segment Reporting (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Income Statement Data Abstract
Net interest income	$ 219,392	$ 190,122	$ 655,625	$ 554,215
Net intersegment interest income (expense)	0	0	0	0
Segment net interest income	219,392	190,122	655,625	554,215
Noninterest income	88,300	85,895	271,522	248,136
Total revenue	307,692	276,017	927,146	802,351
Credit provision	(5,000)	5,000	(1,000)	26,000
Noninterest Expense	204,413	177,427	628,636	527,434
Income (loss) before income taxes	108,279	93,590	299,510	248,917
Income tax expense (benefit)	22,349	28,589	54,932	69,663
Net income	$ 85,929	$ 65,001	$ 244,578	$ 179,254
Return on average allocated capital (ROCET1)	13.20%	11.70%	12.90%	11.00%
Segment Balance Sheet Data
Average earning assets	$ 30,474,870	$ 27,459,795	$ 30,060,938	$ 26,851,992
Average loans	22,973,717	20,899,137	22,689,170	20,500,978
Average deposits	24,695,918	22,441,445	24,001,604	21,813,679
Average allocated capital (CET1)	2,513,272	2,119,675	2,464,354	2,093,962
Allocated Goodwill - Segment			1,168,922	972,006
Business Combination, Acquisition Related Costs		0		0
Operating Segments | Corporate and Commercial Specialty
Segment Income Statement Data Abstract
Net interest income	113,298	93,114	339,718	270,256
Net intersegment interest income (expense)	(13,018)	(758)	(36,151)	1,359
Segment net interest income	100,280	92,356	303,567	271,615
Noninterest income	12,280	12,278	39,156	36,768
Total revenue	112,560	104,634	342,723	308,383
Credit provision	11,232	9,499	32,955	32,549
Noninterest Expense	41,828	39,681	122,853	116,578
Income (loss) before income taxes	59,500	55,454	186,914	159,256
Income tax expense (benefit)	12,098	19,070	36,978	53,082
Net income	$ 47,402	$ 36,384	$ 149,937	$ 106,174
Return on average allocated capital (ROCET1)	15.50%	12.80%	16.60%	12.70%
Segment Balance Sheet Data
Average earning assets	$ 11,981,760	$ 10,923,762	$ 11,814,674	$ 10,846,418
Average loans	11,974,090	10,916,829	11,804,458	10,837,933
Average deposits	8,695,170	7,398,970	8,127,134	6,759,105
Average allocated capital (CET1)	1,215,331	1,125,181	1,207,925	1,121,800
Allocated Goodwill - Segment			524,525	428,000
Operating Segments | Community, Consumer, and Business
Segment Income Statement Data Abstract
Net interest income	91,323	81,058	268,137	236,407
Net intersegment interest income (expense)	21,951	9,894	63,301	33,603
Segment net interest income	113,275	90,952	331,438	270,011
Noninterest income	73,838	67,867	224,109	198,546
Total revenue	187,113	158,819	555,547	468,557
Credit provision	5,280	5,046	15,125	15,317
Noninterest Expense	139,653	120,241	405,338	361,580
Income (loss) before income taxes	42,180	33,532	135,084	91,660
Income tax expense (benefit)	8,861	11,736	28,371	32,081
Net income	$ 33,319	$ 21,796	$ 106,713	$ 59,579
Return on average allocated capital (ROCET1)	20.00%	14.70%	21.90%	13.60%
Segment Balance Sheet Data
Average earning assets	$ 10,456,159	$ 9,608,242	$ 10,333,412	$ 9,418,173
Average loans	10,453,485	9,602,098	10,329,888	9,414,880
Average deposits	13,716,862	11,788,606	13,518,903	11,568,220
Average allocated capital (CET1)	662,017	588,841	652,745	584,774
Allocated Goodwill - Segment			644,397	544,006
Operating Segments | Risk Management and Shared Services
Segment Income Statement Data Abstract
Net interest income	14,770	15,951	47,770	47,552
Net intersegment interest income (expense)	(8,933)	(9,136)	(27,150)	(34,963)
Segment net interest income	5,837	6,814	20,620	12,589
Noninterest income	2,183	5,750	8,257	12,822
Total revenue	8,019	12,564	28,876	25,411
Credit provision	(21,512)	(9,545)	(49,081)	(21,866)
Noninterest Expense	22,932	17,505	100,445	49,276
Income (loss) before income taxes	6,599	4,604	(22,488)	(1,999)
Income tax expense (benefit)	1,390	(2,217)	(10,416)	(15,500)
Net income	$ 5,209	$ 6,821	$ (12,072)	$ 13,501
Return on average allocated capital (ROCET1)	1.80%	4.40%	(4.20%)	2.20%
Segment Balance Sheet Data
Average earning assets	$ 8,036,951	$ 6,927,791	$ 7,912,853	$ 6,587,401
Average loans	546,142	380,210	554,824	248,165
Average deposits	2,283,886	3,253,869	2,355,566	3,486,354
Average allocated capital (CET1)	635,924	$ 405,653	603,684	$ 387,388
Business Combination, Acquisition Related Costs	$ 2,271		$ 29,983