Recent Accounting and Regulatory Pronouncements - Impact of ASU (Details) - USD ($) $ in Thousands		Jan. 01, 2020	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Assets:
Investment Securities - Available for Sale			$ 4,631,554		$ 3,330,672
Investment Securities - Held to Maturity			1,641,485		955,542
Allowance for Credit Losses on Loans			(314,144)	$ (350,401)	(457,309)	$ (440,159)	$ (434,608)	$ (56,927)
Deferred Tax Asset			19,684		110,946
Accrued Interest Receivable - Loans			76,300		93,900
Liabilities:
Reserve for Loan Losses - Unfunded Commitments			314,144		457,309
Equity:
Retained Earnings			925,044		657,451
Credit Discount on Acquired Credit Impaired Loans		$ 3,408
Financing Receivable, Allowance for Credit Losses			314,144	$ 350,401	457,309	440,159	434,608	56,927
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent		22.00%
Non-acquired loans
Assets:
Allowance for Credit Losses on Loans		$ (34,049)
Equity:
Financing Receivable, Allowance for Credit Losses		34,049
Acquired loans
Assets:
Allowance for Credit Losses on Loans		(16,981)
Equity:
Financing Receivable, Allowance for Credit Losses		16,981
Purchased Credit Deteriorated Loans
Assets:
Allowance for Credit Losses on Loans		(3,408)
Equity:
Financing Receivable, Allowance for Credit Losses		3,408
ASU 2016-13
Assets:
Acquired Loans	[1]	1,723
Allowance for Credit Losses on Loans	[2]	(54,438)
Deferred Tax Asset	[3]	12,639
Accrued Interest Receivable - Loans	[1]	1,677
Liabilities:
Reserve for Loan Losses - Unfunded Commitments	[4]	6,421
Equity:
Retained Earnings	[5]	(44,820)
Financing Receivable, Allowance for Credit Losses	[2]	54,438
Other Assets
Assets:
Accrued Interest Receivable - Loans			$ 76,300		$ 93,900
Other Assets | ASU 2016-13
Assets:
Accrued Interest Receivable - Loans		1,677
As previously recorded by acquiree | ASU 2016-13
Assets:
Investment Securities - Available for Sale		1,956,047
Non - Acquired Loans		9,252,831
Acquired Loans		2,118,940
Allowance for Credit Losses on Loans		(111,365)
Deferred Tax Asset		43,955
Accrued Interest Receivable - Loans		30,009
Liabilities:
Reserve for Loan Losses - Unfunded Commitments		6,756
Equity:
Retained Earnings		635,075
Financing Receivable, Allowance for Credit Losses		111,365
Adjustments
Assets:
Allowance for Credit Losses on Loans							(433,066)	(111,365)
Equity:
Financing Receivable, Allowance for Credit Losses							433,066	111,365
Adjustments | ASU 2016-13
Assets:
Investment Securities - Available for Sale		1,956,047
Non - Acquired Loans		9,252,831
Acquired Loans		2,117,209
Allowance for Credit Losses on Loans		(56,927)				(109,442)	1,542	(51,030)
Deferred Tax Asset		31,316
Accrued Interest Receivable - Loans		28,332
Liabilities:
Reserve for Loan Losses - Unfunded Commitments		335
Equity:
Retained Earnings		679,895
Financing Receivable, Allowance for Credit Losses		$ 56,927				$ 109,442	$ (1,542)	$ 51,030

[1] Accrued interest receivable from acquired credit impaired loans of $1,677 was reclassified to other assets and was offset by the reclassification of the grossed up credit discount on acquired credit impaired loans of $3,408 that was moved to the ACL for the purchased credit deteriorated loans.

[2] This is the calculated adjustment to the ACL related to the adoption of ASC 326. Additional reserve related to non-acquired loans was $34,049, to acquired loans was $16,981 and to purchased credit deteriorated loans was $3,408.

[3] This is the effect of deferred tax assets related to the adjustment to the ACL from the adoption of ASC 326 using a 22% tax rate.

[4] This is the adjustment to the reserve for unfunded commitments related to the adoption of ASC 326.

[5] This is the net adjustment to retained earnings related to the adoption of ASC 326.