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Loans and Loans Held for Sale - Additional Information (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) loan tdr	Mar. 31, 2017 loan tdr	Dec. 31, 2017 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	$ 5,100,000		$ 5,200,000
Threshold period of satisfactory performance for troubled debt restructuring to be restored to accruing status	6 months
Number of troubled debt restructuring loans returned back to accruing status | tdr	0	0
Commitment to lend additional funds on troubled debt restructuring	$ 0
Minimum period of loan payment defaults following restructure for TDRs to be in default	90 days
Number of defaulted troubled debt restructuring loans restructured within 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	78.00%		78.00%
Commercial loans	$ 4,491,982,000		$ 4,503,594,000
Commercial real estate | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial loans	2,760,891,000		2,685,994,000
Commercial and industrial | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial loans	1,406,950,000		1,433,266,000
Commercial construction | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial loans	324,141,000		384,334,000
Commercial real estate and commercial construction | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial loans	$ 3,100,000,000		$ 3,100,000,000
Combined percentage of commercial real estate and commercial construction in total commercial loans	69.00%		68.00%
Combined percentage of commercial real estate and commercial construction in total portfolio loans	54.00%		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)	13.00%		14.00%
Out of market exposure of combined portfolio (percent)	5.00%		5.20%
Percentage of total loans out-of-state excluding contiguous states	2.70%		2.80%

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

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Commitment to lend additional funds on TDRs

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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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Number of Defaulted Troubled Debt Restructuring Loans Restructured Within Twelve Months Prior to Defaulting

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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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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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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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