Fair Value of Financial Instruments - Summary of Estimated Fair Values of Financial Instruments not Carried at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
HTM securities	$ 81,995,000	$ 17,217,000
Net loans	61,089,000	44,733,000
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	18,940,000	17,675,000
HTM securities	82,365,000	16,592,000
Non-marketable securities not measured at net asset value	350,000	241,000
Non-marketable securities measured at net asset value	692,000	390,000
Net loans	61,089,000	44,734,000
FHLB and Federal Reserve Bank stock	107,000	61,000
Non-maturity deposits	169,833,000	101,294,000
Time deposits	1,349,000	688,000
Commitments to extend credit	0	0
Carrying Amount \| Short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	97,000	21,000
Carrying Amount \| 3.50% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	349,000	349,000
Carrying Amount \| 3.125% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	496,000	495,000
Carrying Amount \| 1.800% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	495,000
Carrying Amount \| 2.100% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	496,000
Junior subordinated debentures	90,000
Estimate of Fair Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	18,940,000	17,675,000
HTM securities	81,995,000	17,217,000
Non-marketable securities not measured at net asset value	350,000	241,000
Non-marketable securities measured at net asset value	692,000	390,000
Net loans	62,489,000	45,324,000
FHLB and Federal Reserve Bank stock	107,000	61,000
Non-maturity deposits	169,833,000	101,294,000
Time deposits	1,350,000	502,000
Commitments to extend credit	45,000	37,000
Estimate of Fair Value Measurement \| Short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	97,000	21,000
Estimate of Fair Value Measurement \| 3.50% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	376,000	383,000
Estimate of Fair Value Measurement \| 3.125% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	535,000	564,000
Estimate of Fair Value Measurement \| 1.800% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	481,000
Estimate of Fair Value Measurement \| 2.100% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	505,000
Junior subordinated debentures	86,000
Estimate of Fair Value Measurement \| Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	18,940,000	17,675,000
HTM securities	0	0
Non-marketable securities not measured at net asset value	0	0
Non-marketable securities measured at net asset value	0	0
Net loans	0	0
FHLB and Federal Reserve Bank stock	0	0
Non-maturity deposits	169,833,000	101,294,000
Time deposits	0	0
Commitments to extend credit	0	0
Estimate of Fair Value Measurement \| Level 1 \| Short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	0	0
Estimate of Fair Value Measurement \| Level 1 \| 3.50% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	0	0
Estimate of Fair Value Measurement \| Level 1 \| 3.125% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	0	0
Estimate of Fair Value Measurement \| Level 1 \| 1.800% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	0
Estimate of Fair Value Measurement \| Level 1 \| 2.100% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	0
Junior subordinated debentures	0
Estimate of Fair Value Measurement \| Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0	0
HTM securities	81,995,000	17,217,000
Non-marketable securities not measured at net asset value	0	0
Non-marketable securities measured at net asset value	0	0
Net loans	0	0
FHLB and Federal Reserve Bank stock	0	0
Non-maturity deposits	0	0
Time deposits	1,350,000	502,000
Commitments to extend credit	0	0
Estimate of Fair Value Measurement \| Level 2 \| Short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	97,000	21,000
Estimate of Fair Value Measurement \| Level 2 \| 3.50% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	376,000	383,000
Estimate of Fair Value Measurement \| Level 2 \| 3.125% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	535,000	564,000
Estimate of Fair Value Measurement \| Level 2 \| 1.800% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	481,000
Estimate of Fair Value Measurement \| Level 2 \| 2.100% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	505,000
Junior subordinated debentures	86,000
Estimate of Fair Value Measurement \| Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0	0
HTM securities	0	0
Non-marketable securities not measured at net asset value	350,000	241,000
Non-marketable securities measured at net asset value	0	0
Net loans	62,489,000	45,324,000
FHLB and Federal Reserve Bank stock	107,000	61,000
Non-maturity deposits	0	0
Time deposits	0	0
Commitments to extend credit	45,000	37,000
Estimate of Fair Value Measurement \| Level 3 \| Short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	0	0
Estimate of Fair Value Measurement \| Level 3 \| 3.50% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	0	0
Estimate of Fair Value Measurement \| Level 3 \| 3.125% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	0	$ 0
Estimate of Fair Value Measurement \| Level 3 \| 1.800% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	0
Estimate of Fair Value Measurement \| Level 3 \| 2.100% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	0
Junior subordinated debentures	$ 0

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Fair Value, Investments, Entities that Calculate Net Asset Value, Non-Marketable Securities

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The amount shown on the books that a bank with insufficient reserves borrows, at the federal funds rate, from another bank to meet its reserve requirements and the amount of securities that an institution sells and agrees to repurchase at a specified date for a specified price, net of any reductions or offsets. Also reflects the total carrying amount as of the balance sheet date of debt having initial terms less than one year or the normal operating cycle, if longer.

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Investment in Federal Home Loan Bank Stock and Federal Reserve Bank, Fair Value Disclosure

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Non-marketable securities accounted for under cost and equity method accounting

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Fair value portion 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.

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- Definition

Fair value of subordinated debt obligations including, but not limited to, subordinated loan, subordinated bond, subordinated debenture or junior debt.

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

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Amount, after allowance for credit loss, of financing receivable. Excludes financing receivable covered under loss sharing agreement.

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