Fair Value of Financial Instruments, Carrying Amount and Estimated Fair Value of Financial Instruments (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Financial assets [Abstract]
Securities held-to-maturity, fair value	$ 20,829,978	$ 25,233,254
Loans Receivable, Fair Value Disclosure	15,252,953,000	8,178,982,000
Mortgage loans held-for-sale	104,986,000	47,892,000
Loans Held-for-sale, Fair Value Disclosure	25,761,000	22,674,000
Financial liabilities [Abstract]
Deposits and securities sold under agreements to repurchase	16,516,342,000	8,579,664,000
Federal Home Loan Bank advances	1,313,311,000	406,491,000
Subordinated debt and other borrowings	445,098,000	328,049,000
Standby Letters of Credit [Member]
Off-balance sheet instruments [Abstract]
Standby letters of credit	800,000	740,000
Commitments to Extend Credit [Member]
Off-balance sheet instruments [Abstract]
Commitments to extend credit	2,264,000	383,000
Reported Value Measurement [Member]
Financial assets [Abstract]
Securities held-to-maturity, fair value	20,762,000	25,251,000
Loans Receivable, Fair Value Disclosure	15,565,876,000	8,390,944,000
Mortgage loans held-for-sale	103,729,000	47,710,000
Loans Held-for-sale, Fair Value Disclosure	25,456,000	22,588,000
Financial liabilities [Abstract]
Deposits and securities sold under agreements to repurchase	16,586,964,000	8,845,014,000
Federal Home Loan Bank advances	1,319,909,000	406,304,000
Subordinated debt and other borrowings	465,505,000	350,768,000
Reported Value Measurement [Member] \| Standby Letters of Credit [Member]
Off-balance sheet instruments [Abstract]
Commitments to extend credit	5,788,425,000	3,374,269,000
Reported Value Measurement [Member] \| Commitments to Extend Credit [Member]
Off-balance sheet instruments [Abstract]
Standby letters of credit	143,684,000	131,418,000
Estimate of Fair Value Measurement [Member] \| Quoted Market Prices in an Active Market (Level 1) [Member]
Financial assets [Abstract]
Securities held-to-maturity, fair value	0	0
Loans Receivable, Fair Value Disclosure	0	0
Mortgage loans held-for-sale	0	0
Loans Held-for-sale, Fair Value Disclosure	0	0
Financial liabilities [Abstract]
Deposits and securities sold under agreements to repurchase	0	0
Federal Home Loan Bank advances	0	0
Subordinated debt and other borrowings	0	0
Estimate of Fair Value Measurement [Member] \| Quoted Market Prices in an Active Market (Level 1) [Member] \| Standby Letters of Credit [Member]
Off-balance sheet instruments [Abstract]
Standby letters of credit	0	0
Estimate of Fair Value Measurement [Member] \| Quoted Market Prices in an Active Market (Level 1) [Member] \| Commitments to Extend Credit [Member]
Off-balance sheet instruments [Abstract]
Commitments to extend credit	0	0
Estimate of Fair Value Measurement [Member] \| Models with Significant Observable Market Parameters (Level 2) [Member]
Financial assets [Abstract]
Securities held-to-maturity, fair value	20,830,000	25,233,000
Loans Receivable, Fair Value Disclosure	0	0
Mortgage loans held-for-sale	104,986,000	47,892,000
Loans Held-for-sale, Fair Value Disclosure	25,761,000	22,674,000
Financial liabilities [Abstract]
Deposits and securities sold under agreements to repurchase	0	0
Federal Home Loan Bank advances	0	0
Subordinated debt and other borrowings	0	0
Estimate of Fair Value Measurement [Member] \| Models with Significant Observable Market Parameters (Level 2) [Member] \| Standby Letters of Credit [Member]
Off-balance sheet instruments [Abstract]
Standby letters of credit	0	0
Estimate of Fair Value Measurement [Member] \| Models with Significant Observable Market Parameters (Level 2) [Member] \| Commitments to Extend Credit [Member]
Off-balance sheet instruments [Abstract]
Commitments to extend credit	0	0
Estimate of Fair Value Measurement [Member] \| Models with Significant Unobservable Market Parameters (Level 3) [Member]
Financial assets [Abstract]
Securities held-to-maturity, fair value	0	0
Loans Receivable, Fair Value Disclosure	15,252,953,000	8,178,982,000
Mortgage loans held-for-sale	0	0
Loans Held-for-sale, Fair Value Disclosure	0	0
Financial liabilities [Abstract]
Deposits and securities sold under agreements to repurchase	16,516,342,000	8,579,664,000
Federal Home Loan Bank advances	1,313,311,000	406,491,000
Subordinated debt and other borrowings	445,098,000	328,049,000
Estimate of Fair Value Measurement [Member] \| Models with Significant Unobservable Market Parameters (Level 3) [Member] \| Standby Letters of Credit [Member]
Off-balance sheet instruments [Abstract]
Standby letters of credit	800,000	740,000
Estimate of Fair Value Measurement [Member] \| Models with Significant Unobservable Market Parameters (Level 3) [Member] \| Commitments to Extend Credit [Member]
Off-balance sheet instruments [Abstract]
Commitments to extend credit	$ 2,264,000	$ 383,000

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- Definition

The fair value of financial liabilities, which are not recognized in the financial statements (off-balance sheet) because they fail to meet some other criterion for recognition.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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- Definition

Fair value portion of borrowing from Federal Home Loan Bank (FHLBank).

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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- Definition

Fair value of investments in debt securities classified as held-to-maturity.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

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Fair value portion of loans held-for-sale including, but not limited to, mortgage loans and finance receivables.

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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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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Fair value portion of mortgage loans held-for-sale.

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Fair value portion of securities that an entity sells and agrees to repurchase at a specified date for a specified price.

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