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Loans and Loans Held for Sale - Additional Information (Details) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($) loan	Jun. 30, 2017 USD ($) loan	Jun. 30, 2018 USD ($) loan	Jun. 30, 2017 USD ($) loan	Dec. 31, 2017 USD ($)	Mar. 31, 2018 Property
Accounts, Notes, Loans and Financing Receivable [Line Items]
Change-off arising from participation loan agreement	$ 8,632	$ 5,822	$ 10,076	$ 8,763
Loans, net of unearned income	$ 5,400		$ 5,400		$ 5,200
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 | loan	0	1	0	1
Number of commitments to lend additional funds on TDRs | loan	7		7
Commitments to lend additional funds on TDRs	$ 5,600		$ 5,600
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	0	0
Amount of defaulted troubled debt restructuring loans restructured within twelve months prior to defaulting		$ 2,000		$ 2,000
Land
Accounts, Notes, Loans and Financing Receivable [Line Items]
Increase in other real estate			$ 2,500
Other real estate owned, number | Property						2
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of commercial loans in total portfolio loans	79.00%		79.00%		78.00%
Commercial loans	$ 4,544,006		$ 4,544,006		$ 4,503,594
Other consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Change-off arising from participation loan agreement	414	$ 370	872	$ 804
Commercial real estate and commercial construction | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial loans	$ 3,100,000		$ 3,100,000		$ 3,100,000
Combined percentage of commercial real estate and commercial construction in total commercial loans	68.00%		68.00%		68.00%
Combined percentage of commercial real estate and commercial construction in total portfolio loans	53.00%		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)	13.00%		13.00%		14.00%
Out of market exposure of combined portfolio (percent)	5.30%		5.30%		5.20%
Percentage of total loans out-of-state excluding contiguous states	2.80%		2.80%		2.80%
Vehicles and Equipment Liens | Other consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Change-off arising from participation loan agreement	$ 5,200		$ 5,200

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

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Amount of loans and leases that have been written off from both loan receivables and allowance reserve for credit loss.

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