Allowance for Credit Losses - Changes in Allowance for Credit Losses (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable Allowance For Credit Losses [Line Items]
Beginning balance	$ 1,051,071	$ 1,019,444	$ 1,017,198
Provision for credit losses	800,000	176,000	132,000
Net charge-offs
Charge-offs	(333,507)	(237,708)	(231,241)
Recoveries	86,366	93,335	101,487
Net (charge-offs) recoveries	(247,141)	(144,373)	(129,754)
Ending balance	1,736,387	1,051,071	1,019,444
Revision of Prior Period, Accounting Standards Update, Adjustment [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Adoption of new accounting standard	132,457
Commercial, Financial, Leasing, etc. [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Beginning balance	366,094	330,055	328,599
Provision for credit losses	220,544	69,702	33,967
Net charge-offs
Charge-offs	(135,083)	(58,244)	(60,414)
Recoveries	15,765	24,581	27,903
Net (charge-offs) recoveries	(119,318)	(33,663)	(32,511)
Ending balance	405,846	366,094	330,055
Commercial, Financial, Leasing, etc. [Member] | Revision of Prior Period, Accounting Standards Update, Adjustment [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Adoption of new accounting standard	(61,474)
Commercial Real Estate [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Beginning balance	322,201	341,655	374,085
Provision for credit losses	356,203	(10,726)	(41,181)
Net charge-offs
Charge-offs	(35,891)	(12,664)	(12,286)
Recoveries	4,550	3,936	21,037
Net (charge-offs) recoveries	(31,341)	(8,728)	8,751
Ending balance	670,719	322,201	341,655
Commercial Real Estate [Member] | Revision of Prior Period, Accounting Standards Update, Adjustment [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Adoption of new accounting standard	23,656
One-to-Four Family Residential [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Beginning balance	56,033	69,125	65,405
Provision for credit losses	(3,172)	(8,585)	12,401
Net charge-offs
Charge-offs	(10,283)	(12,711)	(15,345)
Recoveries	7,116	8,204	6,664
Net (charge-offs) recoveries	(3,167)	(4,507)	(8,681)
Ending balance	103,590	56,033	69,125
One-to-Four Family Residential [Member] | Revision of Prior Period, Accounting Standards Update, Adjustment [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Adoption of new accounting standard	53,896
Consumer [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Beginning balance	229,118	200,564	170,809
Provision for credit losses	226,425	126,029	127,068
Net charge-offs
Charge-offs	(152,250)	(154,089)	(143,196)
Recoveries	58,935	56,614	45,883
Net (charge-offs) recoveries	(93,315)	(97,475)	(97,313)
Ending balance	556,232	229,118	200,564
Consumer [Member] | Revision of Prior Period, Accounting Standards Update, Adjustment [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Adoption of new accounting standard	194,004
Unallocated [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Beginning balance	77,625	78,045	78,300
Provision for credit losses		(420)	(255)
Net charge-offs
Ending balance		$ 77,625	$ 78,045
Unallocated [Member] | Revision of Prior Period, Accounting Standards Update, Adjustment [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Adoption of new accounting standard	$ (77,625)