Securities - Additional Information (Details)	3 Months Ended	12 Months Ended
Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) Security	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Investments Debt And Equity Securities [Abstract]
Other-than-temporary impairment charges		$ 0	$ 0	$ 0
Carrying value of securities pledged		$ 711,200,000	$ 507,300,000
Percentage of fair value of securities in investment portfolio reflect unrealized loss		84.00%	58.00%
Number of securities in a loss position for more than twelve months \| Security		117
Number of securities in a loss position for less than twelve months \| Security		100
Proceeds from sales of securities available-for-sale	$ 187,800,000	$ 268,799,000	$ 161,401,000	$ 538,960,000
Concentration of tax free municipal securities, percentage	35.00%
Target concentration of tax free municipal securities, percentage	25.00%

Concentration percentage of tax free municipal securities.

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- Definition

Percentage of fair value of securities in investment portfolio reflect unrealized loss.

+ References

No definition available.

+ Details

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- Definition

Target concentration percentage of tax free municipal securities.

+ References

No definition available.

+ Details

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- Definition

Number of investment positions in available-for-sale investments in a continuous unrealized loss position for less than one year or normal operating cycle, if longer, for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

+ References

No definition available.

+ Details

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- Definition

The fair value of available-for-sale securities that serve as collateral for borrowings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5086-111524

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- Definition

Number of investment positions in available-for-sale investments that have been in a continuous unrealized loss position for greater than or equal to a year for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

+ References

No definition available.

+ Details

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- References

No definition available.

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- Definition

The amount of other than temporary impairment losses on investments in debt and equity securities categorized as Available-for-sale which have been recognized in earnings during the period. Such impairment losses consist of other than temporary impairments (OTTI) on equity securities, credit losses on investments in debt securities, and when the entity intends to sell an impaired debt security or it is more likely than not that the entity will be required to sell the impaired debt security before recovery of its amortized cost basis.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 8A
-URI http://asc.fasb.org/extlink&oid=82896770&loc=SL6284422-111562

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- Definition

The cash inflow associated with the sale of debt and equity securities classified as available-for-sale securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26853-111562

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 12
-Subparagraph (a),(b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3179-108585

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