Fair Value Measurements (Details 4) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Valuation Methodology
Investment securities, available-for-sale	$ 942,076	$ 904,033
Level 3
Valuation Methodology
Investment securities, available-for-sale	2,863	2,865
Recurring \| Level 3
Valuation Methodology
Investment securities, available-for-sale	2,863	2,865
Recurring \| Level 3 \| Discounted cash flows \| Direct placement municipal securities
Valuation Methodology
Investment securities, available-for-sale	$ 2,155	$ 2,155
Recurring \| Level 3 \| Discounted cash flows \| Direct placement municipal securities \| Minimum
Unobservable Inputs
Credit spread assumption (as a percent)	2.26%	2.21%
Recurring \| Level 3 \| Discounted cash flows \| Direct placement municipal securities \| Maximum
Unobservable Inputs
Credit spread assumption (as a percent)	3.08%	2.93%
Recurring \| Level 3 \| Discounted cash flows \| Foreign government
Valuation Methodology
Investment securities, available-for-sale	$ 708	$ 710
Recurring \| Level 3 \| Discounted cash flows \| Foreign government \| Minimum
Unobservable Inputs
Market yield assumption (as a percent)	0.22%	0.35%
Recurring \| Level 3 \| Discounted cash flows \| Foreign government \| Maximum
Unobservable Inputs
Market yield assumption (as a percent)	1.33%	1.23%
Non-recurring \| Level 3 \| Commercial loans
Unobservable Inputs
Discount rate (as a percent)	20.00%
Non-recurring \| Level 3 \| Minimum \| Commercial loans
Unobservable Inputs
Discount rate (as a percent)	40.00%
Non-recurring \| Level 3 \| Maximum \| Commercial loans
Unobservable Inputs
Discount rate (as a percent)	75.00%
Non-recurring \| Level 3 \| Trade publications \| Aircraft
Unobservable Inputs
Discount rate (as a percent)	10.00%
Non-recurring \| Level 3 \| Auction values \| Autos
Unobservable Inputs
Discount rate (as a percent)	10.00%
Non-recurring \| Level 3 \| Trade publications and auction values \| Medium and heavy duty trucks
Unobservable Inputs
Discount rate (as a percent)	15.00%
Non-recurring \| Level 3 \| Trade publications and auction values \| Construction equipment
Unobservable Inputs
Discount rate (as a percent)	20.00%
Non-recurring \| Level 3 \| Appraisals \| Real estate
Unobservable Inputs
Discount rate (as a percent)	20.00%

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562

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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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Percentage of adjustment to the observed price for a similar asset or liability when an identical transaction does not exist, used as an input to measure fair value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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Adjustment to the discount rate used to measure fair value for lack of ability to convert business interests into cash quickly.

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Risk associated with risk of default, credit spread risk and downgrade risk, expressed as a percentage, used as an input to measure fair value.

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Percentage of likelihood a loan will not be repaid and instead default, used as an input to measure fair value.

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