Revenue from Contracts with Customers (Details) - USD ($) $ in Thousands		3 Months Ended			9 Months Ended
	Jun. 30, 2020	Sep. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Revenue [Line Items]
Noninterest income (in-scope of Topic 606)		$ 53,099		$ 49,243	$ 158,383	$ 187,524
Noninterest income (out-of-scope of Topic 606)(b)		28,977		26,303	92,479	240,818
Total noninterest income		82,076		75,545	250,862	428,342
Whitnell
Disaggregation of Revenue [Line Items]
Gain (Loss) on Sale and Maturity of Other Investments, Before Tax			$ 2,000		2,000
ABRC
Disaggregation of Revenue [Line Items]
Pre-tax book gain on sale	$ 163,000
Wealth management fees
Disaggregation of Revenue [Line Items]
Noninterest income (in-scope of Topic 606)		22,110		21,152	67,229	62,884
Service charges and deposit account fees
Disaggregation of Revenue [Line Items]
Noninterest income (in-scope of Topic 606)		16,962		14,283	47,366	40,989
Card-based fees(a)
Disaggregation of Revenue [Line Items]
Noninterest income (in-scope of Topic 606)		11,147		10,207	31,920	28,736
Insurance commissions and fees
Disaggregation of Revenue [Line Items]
Noninterest income (in-scope of Topic 606)		88		114	250	45,153
Other revenue
Disaggregation of Revenue [Line Items]
Noninterest income (in-scope of Topic 606)		2,792		3,487	11,618	9,764
Corporate and Commercial Specialty
Disaggregation of Revenue [Line Items]
Noninterest income (in-scope of Topic 606)		27,998		27,540	85,212	77,690
Noninterest income (out-of-scope of Topic 606)(b)		15,230		9,866	39,635	33,564
Corporate and Commercial Specialty | Wealth management fees
Disaggregation of Revenue [Line Items]
Noninterest income (in-scope of Topic 606)		22,110		21,152	67,229	61,496
Corporate and Commercial Specialty | Service charges and deposit account fees
Disaggregation of Revenue [Line Items]
Noninterest income (in-scope of Topic 606)		4,539		4,458	13,898	12,316
Corporate and Commercial Specialty | Card-based fees(a)
Disaggregation of Revenue [Line Items]
Noninterest income (in-scope of Topic 606)		541		474	1,367	1,216
Corporate and Commercial Specialty | Insurance commissions and fees
Disaggregation of Revenue [Line Items]
Noninterest income (in-scope of Topic 606)		36		57	98	179
Corporate and Commercial Specialty | Other revenue
Disaggregation of Revenue [Line Items]
Noninterest income (in-scope of Topic 606)		772		1,400	2,619	2,481
Community, Consumer, and Business
Disaggregation of Revenue [Line Items]
Noninterest income (in-scope of Topic 606)		25,012		21,739	71,583	109,713
Noninterest income (out-of-scope of Topic 606)(b)		10,758		11,219	41,976	30,728
Community, Consumer, and Business | Wealth management fees
Disaggregation of Revenue [Line Items]
Noninterest income (in-scope of Topic 606)		0		0	0	1,387
Community, Consumer, and Business | Service charges and deposit account fees
Disaggregation of Revenue [Line Items]
Noninterest income (in-scope of Topic 606)		12,417		9,816	33,441	28,653
Community, Consumer, and Business | Card-based fees(a)
Disaggregation of Revenue [Line Items]
Noninterest income (in-scope of Topic 606)		10,600		9,699	30,538	27,392
Community, Consumer, and Business | Insurance commissions and fees
Disaggregation of Revenue [Line Items]
Noninterest income (in-scope of Topic 606)		50		55	146	44,965
Community, Consumer, and Business | Other revenue
Disaggregation of Revenue [Line Items]
Noninterest income (in-scope of Topic 606)		1,945		2,168	7,459	7,315
Risk Management and Shared Services
Disaggregation of Revenue [Line Items]
Noninterest income (in-scope of Topic 606)		89		(36)	1,588	121
Noninterest income (out-of-scope of Topic 606)(b)		2,989		5,218	10,868	176,525
Risk Management and Shared Services | Service charges and deposit account fees
Disaggregation of Revenue [Line Items]
Noninterest income (in-scope of Topic 606)		5		10	27	19
Risk Management and Shared Services | Card-based fees(a)
Disaggregation of Revenue [Line Items]
Noninterest income (in-scope of Topic 606)		6		34	15	127
Risk Management and Shared Services | Insurance commissions and fees
Disaggregation of Revenue [Line Items]
Noninterest income (in-scope of Topic 606)		2		2	6	8
Risk Management and Shared Services | Other revenue
Disaggregation of Revenue [Line Items]
Noninterest income (in-scope of Topic 606)		76		(82)	1,540	(33)
Operating Segments [Member] | Corporate and Commercial Specialty
Disaggregation of Revenue [Line Items]
Total noninterest income		43,228		37,405	124,847	111,254
Operating Segments [Member] | Community, Consumer, and Business
Disaggregation of Revenue [Line Items]
Total noninterest income		35,771		32,958	113,558	140,442
Operating Segments [Member] | Risk Management and Shared Services
Disaggregation of Revenue [Line Items]
Total noninterest income		$ 3,077		$ 5,182	$ 12,457	$ 176,646