Business Segment Information - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Fixed income	$ 110,943	$ 77,645	$ 318,999	$ 197,808
Mortgage banking and title income	65,503	2,019	72,072	6,477
Deposit transactions and cash management	42,056	34,379	103,133	98,374
Brokerage, management fees and commissions	17,891	14,157	47,096	40,910
Trust services and investment management	11,741	7,163	26,669	22,077
Bankcard income	10,457	7,017	24,362	20,324
Securities gains (losses), net	(1,170)	97	(2,638)	177
Purchase accounting gain	532,150	0	532,150	0
Other income	33,352	29,258	82,105	84,626
Total noninterest income	822,923	171,735	1,203,948	470,773
Regional Banking
Disaggregation of Revenue [Line Items]
Fixed income	380	57	582	120
Mortgage banking and title income	65,569	1,643	71,168	5,119
Deposit transactions and cash management	40,500	32,624	98,906	93,316
Brokerage, management fees and commissions	19,595	14,157	48,799	40,910
Trust services and investment management	14,599	7,190	29,562	22,148
Bankcard income	10,150	7,030	23,853	20,670
Securities gains (losses), net	0	0	0	0
Purchase accounting gain	0		0	0
Other income	21,664	23,229	60,799	58,272
Total noninterest income	172,457	85,930	333,669	240,555
Fixed Income
Disaggregation of Revenue [Line Items]
Fixed income	110,563	77,588	318,417	196,582
Mortgage banking and title income	0	0	0	0
Deposit transactions and cash management	0	0	0	4
Brokerage, management fees and commissions	0	0	0	0
Trust services and investment management	0	0	0	0
Bankcard income	0	11	0	11
Securities gains (losses), net	0	0	0	0
Purchase accounting gain	0		0	0
Other income	220	209	1,324	641
Total noninterest income	110,783	77,808	319,741	197,238
Corporate
Disaggregation of Revenue [Line Items]
Fixed income	0	0	0	0
Mortgage banking and title income	23	0	23	0
Deposit transactions and cash management	1,528	1,721	4,131	4,956
Brokerage, management fees and commissions	(1,704)	0	(1,703)	0
Trust services and investment management	(2,858)	(27)	(2,893)	(71)
Bankcard income	277	63	421	185
Securities gains (losses), net	(1,170)	97	(2,638)	177
Purchase accounting gain	532,150		532,150	0
Other income	11,422	5,505	19,400	24,865
Total noninterest income	539,668	7,359	548,891	30,112
Non-Strategic
Disaggregation of Revenue [Line Items]
Fixed income	0	0	0	1,106
Mortgage banking and title income	(89)	376	881	1,358
Deposit transactions and cash management	28	34	96	98
Brokerage, management fees and commissions	0	0	0	0
Trust services and investment management	0	0	0	0
Bankcard income	30	(87)	88	(542)
Securities gains (losses), net	0	0	0	0
Purchase accounting gain	0		0	0
Other income	46	315	582	848
Total noninterest income	15	638	1,647	2,868
Underwriting, portfolio advisory, and other noninterest income
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	$ 9,900	$ 9,200	$ 28,600	$ 23,600