Mortgage Servicing Rights - Data and Key Economic Assumptions Related to the Company's MRSs (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Data and key economic assumptions related to mortgage servicing rights
Unpaid principal balance	$ 4,089,045	$ 4,012,519
Weighted-average prepayment speed (CPR)	7.42%	8.04%
Estimated impact of a 10% increase	$ (1,692)	$ (1,592)
Estimated impact of a 20% increase	$ (3,280)	$ (3,095)
Discount rate	9.40%	9.69%
Estimated impact of a 10% increase	$ (2,374)	$ (2,027)
Estimated impact of a 20% increase	$ (4,556)	$ (3,896)
Weighted-average coupon interest rate	3.89%	3.89%
Weighted-average servicing fee	26.54%	26.36%
Weighted-average remaining maturity	8 years 3 months 7 days	7 years 11 months 23 days

Data and key economic assumptions related to mortgage servicing rights.

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- Definition

Value assumptions used to estimate fair value unpaid principle balance.

+ References

No definition available.

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- Definition

Servicing assets and servicing liabilities at fair value assumptions used to estimate fair value weighted average coupon interest rate.

+ References

No definition available.

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- Definition

Servicing assets and servicing liabilities at fair value assumptions used to estimate fair value weighted average remaining maturity.

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No definition available.

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- Definition

Servicing assets and servicing liabilities at fair value assumptions used to estimate fair value weighted average servicing fee.

+ References

No definition available.

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- Definition

The hypothetical financial impact of a 10 percent adverse change of the discount rate on the fair value of transferor's interests in transferred financial assets (including any servicing assets or servicing liabilities) as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82924980&loc=d3e107314-111719

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- Definition

The hypothetical financial impact of a 10 percent adverse change of prepayment speed on the fair value of transferor's interests in transferred financial assets (including any servicing assets or servicing liabilities) as of the balance sheet date.

+ References

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- Definition

The hypothetical financial impact of a 20 percent adverse change of the discount rate on the fair value of transferor's interests in transferred financial assets (including any servicing assets or servicing liabilities) as of the balance sheet date.

+ References

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- Definition

The hypothetical financial impact of a 20 percent adverse change of prepayment speed on the fair value of transferor's interests in transferred financial assets (including any servicing assets or servicing liabilities) as of the balance sheet date.

+ References

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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-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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- Definition

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

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