Fair Value Measurements - Carrying value and estimated fair value (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Assets:
Cash and cash equivalents	$ 15,076,845	$ 6,938,698	$ 6,473,599
Debt securities held to maturity	9,444,036	6,797,046
Debt securities held to maturity, estimated fair value	9,779,195	6,921,158
Liabilities:
Deposits	86,371,032	74,985,283
Fair Value, Inputs, Level 1
Assets:
Cash and cash equivalents, estimated fair value	15,076,845	6,938,698
Debt securities held to maturity, estimated fair value	1,412,275	1,340,448
Fair Value, Inputs, Level 2
Assets:
Debt securities held to maturity, estimated fair value	7,838,912	4,912,399
Liabilities:
Deposits, estimated fair value	86,400,715	75,024,350
FHLB and other borrowings, estimated fair value	3,444,859	3,721,949
Federal funds purchased and securities sold under agreements to repurchase, estimated fair value	189,474	173,028
Fair Value, Inputs, Level 3
Assets:
Debt securities held to maturity, estimated fair value	528,008	668,311
Loans, net, estimated fair value	65,384,443	60,869,662
Reported value measurement
Assets:
Cash and cash equivalents	15,076,845	6,938,698
Debt securities held to maturity	9,444,036	6,797,046
Loans	66,180,612	63,946,857
Liabilities:
Deposits	86,371,032	74,985,283
FHLB and other borrowings	3,560,973	3,690,044
Federal funds purchased and securities sold under agreements to repurchase	189,474	173,028
Estimate of fair value measurement
Assets:
Cash and cash equivalents, estimated fair value	15,076,845	6,938,698
Debt securities held to maturity, estimated fair value	9,779,195	6,921,158
Loans, net, estimated fair value	65,384,443	60,869,662
Liabilities:
Deposits, estimated fair value	86,400,715	75,024,350
FHLB and other borrowings, estimated fair value	3,444,859	3,721,949
Federal funds purchased and securities sold under agreements to repurchase, estimated fair value	$ 189,474	$ 173,028

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Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

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The aggregate of all deposit liabilities held by the entity, including foreign and domestic, interest and noninterest bearing; may include demand deposits, saving deposits, Negotiable Order of Withdrawal (NOW) and time deposits among others.

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Fair value portion of deposit liabilities held by the entity, including, but not limited to, foreign and domestic, interest and noninterest bearing, demand deposits, saving deposits, negotiable orders of withdrawal (NOW) and time deposits.

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Fair value portion of short term borrowing where a bank borrows, at the federal funds rate, from another bank and securities that an entity sells and agrees to repurchase at a specified date for a specified price.

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Amount after offset of short term borrowing where a bank borrows, at the federal funds rate, from another bank and securities that an entity sells and agrees to repurchase at a specified date for a specified price.

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Amount of investment in debt security measured at amortized cost (held-to-maturity).

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Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

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Fair value portion of loan receivable, including, but not limited to, mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts.

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