Servicing Rights - Key Assumptions and the Sensitivity of the Fair Value (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Servicing Assets at Fair Value [Line Items]
Aggregate portfolio principal balance	$ 586,595	$ 557,583
Base
Mortgage servicing rights	$ 11,254	$ 10,821
MSR as a percentage of serviced loans	1.92%	1.94%
Mortgage servicing rights
Servicing Assets at Fair Value [Line Items]
Aggregate portfolio principal balance	$ 586,595	$ 557,583
Weighted average rate of note	3.96%	3.91%
Base
Conditional prepayment rate	8.35%	7.70%
Discount rate	9.60%	9.94%
Mortgage servicing rights	$ 11,254	$ 10,821	$ 8,039	$ 7,305
MSR as a percentage of serviced loans	1.92%	1.94%
1.0% Adverse Rate Change
Conditional prepayment rate	20.51%	19.35%
Discount rate	8.60%	8.94%
Fair value MSR	$ 7,299	$ 7,115
Percentage of MSR	1.24%	1.28%
2.0% Adverse Rate Change
Conditional prepayment rate	22.11%	20.95%
Discount rate	7.60%	7.94%
Fair value MSR	$ 7,000	$ 6,829
Percentage of MSR	1.19%	1.22%

Mortgage Loans Serviced by Third Party, Weighted Average Rate of Note

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Servicing Assets And Servicing Liabilities At Fair Value, Assumptions Used To Estimate Fair Value, 1.0% Adverse Rate Change [Abstract]

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Fair value of an asset representing net future revenue from contractually specified servicing fees, late charges, and other ancillary revenues, in excess of future costs related to servicing arrangements.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6469459&loc=d3e122501-111745

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=68078379&loc=d3e122625-111746

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Discount rate used to estimate the fair value of servicing assets and servicing liabilities.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 50
-Paragraph 2
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=68078379&loc=d3e122596-111746

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Estimated rate of prepayments of principal on servicing assets and servicing liabilities.

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