Loans and Loans Held for Sale - Additional Information (Details) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2019 USD ($) loan	Mar. 31, 2018 loan	Dec. 31, 2018 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	$ 4,700		$ 5,300
Threshold period of satisfactory performance for troubled debt restructuring to be restored to accruing status	6 months
Number of troubled debt restructuring loans returned to accruing status \| loan	3	0
Amount of trouble debt restructuring loans returned to accruing status	$ 1,700
Number of commitments to lend additional funds on TDRs \| loan	14
Commitments to lend additional funds on TDRs	$ 13,000
Minimum period of loan payment defaults following restructure for TDRs to be in default	90 days
Number of defaulted TDRs that were restructured within the last twelve months prior to defaulting \| loan	0	0
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of commercial loans in total portfolio loans	79.00%		79.00%
Commercial loans	$ 4,660,290		$ 4,672,445
Commercial real estate and commercial construction \| Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial loans	$ 3,100,000		$ 3,200,000
Combined percentage of commercial real estate and commercial construction in total commercial loans	68.00%		68.00%
Combined percentage of commercial real estate and commercial construction in total portfolio loans	53.00%		53.00%
Concentration risk percentage of commercial real estate and commercial construction	0.00%		0.00%
Maximum concentration of commercial real estate and commercial construction portfolio in loans (in excess of)	13.20%		14.00%
Out of market exposure of combined portfolio (percent)	5.70%		5.40%
Percentage of total loans out-of-state excluding contiguous states	3.00%		2.90%

Combined percentage of commercial real estate and commercial construction in commercial loans.

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Combined percentage of commercial real estate and commercial construction in portfolio loans.

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Commitment To Lend Additional Funds

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Financial receivable trouble debt restructuring reclassified to accruing trouble debt restructuring status.

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Maximum concentration of commercial real estate and commercial construction portfolio in loans.

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Net loans out of state percentage excluding contiguous states.

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Number of troubled debt restructuring loans returned back to accruing status.

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Commitment To Lend Additional Funds

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Out of state exposure of combined portfolio.

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Percentage of commercial loans in portfolio loans.

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Period of loans in default treated as troubled debt restructuring.

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Threshold Period of Satisfactory Payments to Remove from Nonaccrual Status

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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Number of financing receivables that have been modified by troubled debt restructurings within the previous 12 months and for which there was a payment default.

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Amount of deferred interest and fee income, unamortized costs incurred to originate loans and leases, unamortized loan commitments and loan syndication fees, and premiums over or discounts from face amounts of acquired loans. Excludes amounts for loans and leases covered under loss sharing agreements.

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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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