Loans Changes in the Allowance for Loan Losses by Oil and Gas Segment (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2018	Dec. 31, 2017
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period	$ 265,880	$ 278,335
Charge offs	(27,081)	(56,246)
Recoveries	9,302	16,791
Net Charge offs	(17,779)	(39,455)
Provision for loan losses	4,500	27,000
Balance at end of period	252,601	265,880
Allowance for loan losses
Individually evaluated			$ 32,497	$ 25,846
Collectively evaluated			220,104	240,034
Total allowance for loan losses	265,880	265,880	252,601	265,880
Loans
Individually evaluated			211,687	247,575
Collectively evaluated			22,749,199	20,537,416
Total loans			22,976,786	20,784,991
Oil and Gas Portfolio Segment
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period	27,000	38,000
Charge offs	(14,000)	(25,000)
Recoveries	3,000	0
Net Charge offs	(11,000)	(25,000)
Provision for loan losses	1,000	14,000
Balance at end of period	17,000	27,000
Allowance for loan losses
Individually evaluated			9,000	5,000
Collectively evaluated			8,000	22,000
Total allowance for loan losses	$ 27,000	$ 38,000	17,000	27,000
Loans
Individually evaluated			45,000	77,000
Collectively evaluated			637,000	523,000
Total loans			$ 682,000	$ 600,000

Loans.

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Amount of recovery of loans and lease receivables which had previously been fully or partially written-off as bad debts.

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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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Amount, after recoveries, of loans and leases that have been written-off.

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The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

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Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

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Amount before allowance and after deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements.

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Amount of expense related to estimated loss from loan and lease transactions.

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