Loans and Allowance for Credit Losses (Allowance for Credit Losses by Portfolio Class) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses: Beginning balance	$ 450,177	$ 191,251
Allowance for loan losses: Cumulative effect of change in accounting principle		27,330
Allowance for loan losses: Net provision for loan losses	(4,911)	246,793
Allowance for loan losses: Ending balance	424,360	426,003
Reserve for unfunded lending commitments: Beginning balance	29,907	3,974
Reserve for unfunded lending commitments: Provision for losses	2,652	17,688
Reserve for unfunded lending commitments: Ending balance	32,559	48,992
Total allowance for credit losses	456,919	474,995
Allowance for loan losses: Individually evaluated	7,074	68,494
Allowance for loan losses: Collectively evaluated	417,286	357,509
Reserve for unfunded lending commitments: Individually evaluated	241	7,215
Reserve for unfunded lending commitments: Collectively evaluated	32,318	41,777
Loans: Individually evaluated for impairment	42,693	274,529
Loans: Collectively evaluated for impairment	21,622,166	21,241,152
Loans receivable	21,664,859	21,515,681	$ 21,789,931
Non-Purchased Credit Impaired Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses: Cumulative effect of change in accounting principle		49,411
Allowance for loan losses: Charge-offs	(22,104)	(47,738)
Allowance for loan losses: Recoveries	3,850	3,974
Allowance for loan losses: Net provision for loan losses	(7,563)	229,105
Total Commercial [Member] | Commercial Real Estate - Income Producing [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses: Beginning balance	109,474	20,869
Allowance for loan losses: Cumulative effect of change in accounting principle		449
Allowance for loan losses: Ending balance	116,179	64,960
Reserve for unfunded lending commitments: Beginning balance	1,099
Reserve for unfunded lending commitments: Provision for losses	439	280
Reserve for unfunded lending commitments: Ending balance	1,538	729
Total allowance for credit losses	117,717	65,689
Allowance for loan losses: Individually evaluated	22	458
Allowance for loan losses: Collectively evaluated	116,157	64,502
Reserve for unfunded lending commitments: Collectively evaluated	1,538	729
Loans: Individually evaluated for impairment	4,363	7,300
Loans: Collectively evaluated for impairment	3,406,665	3,225,483
Loans receivable	3,411,028	3,232,783	3,357,939
Total Commercial [Member] | Construction and Land Development [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses: Beginning balance	26,462	9,350
Allowance for loan losses: Cumulative effect of change in accounting principle		15,658
Allowance for loan losses: Ending balance	25,173	33,823
Reserve for unfunded lending commitments: Beginning balance	22,694
Reserve for unfunded lending commitments: Provision for losses	(617)	13,205
Reserve for unfunded lending commitments: Ending balance	22,077	28,863
Total allowance for credit losses	47,250	62,686
Allowance for loan losses: Individually evaluated	21	23
Allowance for loan losses: Collectively evaluated	25,152	33,800
Reserve for unfunded lending commitments: Collectively evaluated	22,077	28,863
Loans: Individually evaluated for impairment	131	3,350
Loans: Collectively evaluated for impairment	1,122,010	1,095,376
Loans receivable	1,122,141	1,098,726	1,065,057
Total Commercial [Member] | Non-Purchased Credit Impaired Loans [Member] | Commercial Real Estate - Income Producing [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses: Cumulative effect of change in accounting principle		7,287
Allowance for loan losses: Charge-offs	(194)	(830)
Allowance for loan losses: Recoveries		7
Allowance for loan losses: Net provision for loan losses	6,899	37,627
Total Commercial [Member] | Non-Purchased Credit Impaired Loans [Member] | Construction and Land Development [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses: Cumulative effect of change in accounting principle		7,478
Allowance for loan losses: Charge-offs	(248)
Allowance for loan losses: Recoveries	159	234
Allowance for loan losses: Net provision for loan losses	(1,200)	16,761
Total Commercial [Member] | Total Commercial And Industrial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses: Beginning balance	218,827	117,409
Allowance for loan losses: Cumulative effect of change in accounting principle		6,060
Allowance for loan losses: Ending balance	195,459	235,833
Reserve for unfunded lending commitments: Beginning balance	4,910	3,974
Reserve for unfunded lending commitments: Provision for losses	2,773	5,471
Reserve for unfunded lending commitments: Ending balance	7,683	15,505
Total allowance for credit losses	203,142	251,338
Allowance for loan losses: Individually evaluated	5,806	67,506
Allowance for loan losses: Collectively evaluated	189,653	168,327
Reserve for unfunded lending commitments: Individually evaluated	241	7,215
Reserve for unfunded lending commitments: Collectively evaluated	7,442	8,290
Loans: Individually evaluated for impairment	30,179	257,974
Loans: Collectively evaluated for impairment	12,856,267	11,794,686
Loans receivable	12,886,446	12,052,660	12,844,428
Total Commercial [Member] | Total Commercial And Industrial [Member] | Commercial Non-Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses: Beginning balance	149,693	106,432
Allowance for loan losses: Cumulative effect of change in accounting principle		5,772
Allowance for loan losses: Ending balance	128,936	186,998
Reserve for unfunded lending commitments: Beginning balance	4,529	3,974
Reserve for unfunded lending commitments: Provision for losses	2,642	5,182
Reserve for unfunded lending commitments: Ending balance	7,171	14,928
Total allowance for credit losses	136,107	201,926
Allowance for loan losses: Individually evaluated	4,564	67,404
Allowance for loan losses: Collectively evaluated	124,372	119,594
Reserve for unfunded lending commitments: Individually evaluated	190	7,215
Reserve for unfunded lending commitments: Collectively evaluated	6,981	7,713
Loans: Individually evaluated for impairment	20,132	253,790
Loans: Collectively evaluated for impairment	10,071,210	9,067,550
Loans receivable	10,091,342	9,321,340	9,986,983
Total Commercial [Member] | Total Commercial And Industrial [Member] | Commercial Real Estate - Owner Occupied [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses: Beginning balance	69,134	10,977
Allowance for loan losses: Cumulative effect of change in accounting principle		288
Allowance for loan losses: Ending balance	66,523	48,835
Reserve for unfunded lending commitments: Beginning balance	381
Reserve for unfunded lending commitments: Provision for losses	131	289
Reserve for unfunded lending commitments: Ending balance	512	577
Total allowance for credit losses	67,035	49,412
Allowance for loan losses: Individually evaluated	1,242	102
Allowance for loan losses: Collectively evaluated	65,281	48,733
Reserve for unfunded lending commitments: Individually evaluated	51
Reserve for unfunded lending commitments: Collectively evaluated	461	577
Loans: Individually evaluated for impairment	10,047	4,184
Loans: Collectively evaluated for impairment	2,785,057	2,727,136
Loans receivable	2,795,104	2,731,320	2,857,445
Total Commercial [Member] | Total Commercial And Industrial [Member] | Non-Purchased Credit Impaired Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses: Cumulative effect of change in accounting principle		14,633
Allowance for loan losses: Charge-offs	(17,859)	(41,227)
Allowance for loan losses: Recoveries	1,936	2,307
Allowance for loan losses: Net provision for loan losses	(7,445)	142,711
Total Commercial [Member] | Total Commercial And Industrial [Member] | Non-Purchased Credit Impaired Loans [Member] | Commercial Non-Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses: Cumulative effect of change in accounting principle		(244)
Allowance for loan losses: Charge-offs	(17,512)	(40,713)
Allowance for loan losses: Recoveries	1,899	2,226
Allowance for loan losses: Net provision for loan losses	(5,144)	119,297
Total Commercial [Member] | Total Commercial And Industrial [Member] | Non-Purchased Credit Impaired Loans [Member] | Commercial Real Estate - Owner Occupied [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses: Cumulative effect of change in accounting principle		14,877
Allowance for loan losses: Charge-offs	(347)	(514)
Allowance for loan losses: Recoveries	37	81
Allowance for loan losses: Net provision for loan losses	(2,301)	23,414
Residential Mortgages [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses: Beginning balance	48,842	20,331
Allowance for loan losses: Cumulative effect of change in accounting principle		17
Allowance for loan losses: Ending balance	42,519	48,200
Reserve for unfunded lending commitments: Beginning balance	19
Reserve for unfunded lending commitments: Provision for losses	3
Reserve for unfunded lending commitments: Ending balance	22	17
Total allowance for credit losses	42,541	48,217
Allowance for loan losses: Individually evaluated	486	312
Allowance for loan losses: Collectively evaluated	42,033	47,888
Reserve for unfunded lending commitments: Collectively evaluated	22	17
Loans: Individually evaluated for impairment	5,241	4,625
Loans: Collectively evaluated for impairment	2,483,551	2,975,360
Loans receivable	2,488,792	2,979,985	2,665,212
Residential Mortgages [Member] | Non-Purchased Credit Impaired Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses: Cumulative effect of change in accounting principle		12,921
Allowance for loan losses: Charge-offs	(109)	(141)
Allowance for loan losses: Recoveries	206	212
Allowance for loan losses: Net provision for loan losses	(6,420)	14,877
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses: Beginning balance	46,572	23,292
Allowance for loan losses: Cumulative effect of change in accounting principle		5,146
Allowance for loan losses: Ending balance	45,030	43,187
Reserve for unfunded lending commitments: Beginning balance	1,185
Reserve for unfunded lending commitments: Provision for losses	54	(1,268)
Reserve for unfunded lending commitments: Ending balance	1,239	3,878
Total allowance for credit losses	46,269	47,065
Allowance for loan losses: Individually evaluated	739	195
Allowance for loan losses: Collectively evaluated	44,291	42,992
Reserve for unfunded lending commitments: Collectively evaluated	1,239	3,878
Loans: Individually evaluated for impairment	2,779	1,280
Loans: Collectively evaluated for impairment	1,753,673	2,150,247
Loans receivable	1,756,452	2,151,527	$ 1,857,295
Consumer [Member] | Non-Purchased Credit Impaired Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses: Cumulative effect of change in accounting principle		7,092
Allowance for loan losses: Charge-offs	(3,694)	(5,540)
Allowance for loan losses: Recoveries	1,549	1,214
Allowance for loan losses: Net provision for loan losses	$ 603	$ 17,129