Fair Value of Financial Instruments - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Fair Value Measurements
Transfers between Level 1 and Level 2	$ 0
Transfers between Level 2 and Level 1	0
Assets
Mortgage servicing rights	$ 181,005	$ 154,405	$ 131,537
Level 3
Fair Value Measurements
Percentage of level three assets to total assets measured at fair value	91.00%	87.00%
Percentage of level three liabilities to total liabilities measured at fair value	99.00%	99.00%
Recurring basis \| Level 2
Assets
Mortgage loans held for-for-sale	$ 344,681	$ 568,781
Derivatives assets, lending, net	534	420
Total assets at fair value	345,215	569,201
Recurring basis \| Level 3
Assets
Derivatives assets, lending, net	3,818	4,357
Mortgage servicing rights	181,005	154,405
Securitized mortgage collateral	3,302,192	3,662,008
Total assets at fair value	3,487,015	3,820,770
Liabilities
Securitized mortgage borrowings	3,296,242	3,653,265
Long-term debt	46,738	44,982
Contingent consideration		554
Total liabilities at fair value	$ 3,342,980	$ 3,698,801

Represents the percentage of Level 3 assets measured at estimated fair value to total assets measured at fair value.

No definition available.

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- Definition

Represents the percentage of Level 3 liabilities measured at estimated fair value to total liabilities measured at fair value.

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- Definition

Fair value portion of securitized mortgage borrowings.

+ References

No definition available.

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Fair value portion of securitized mortgage collateral.

+ References

No definition available.

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- Definition

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ 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 (a)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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Fair value of contingent consideration in a business combination that is classified in shareholders' equity.

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Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

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- Definition

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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No definition available.

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Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

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Amount of transfers of assets measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1.

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Fair value of financial and nonfinancial obligations.

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- Definition

The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

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Fair value portion of mortgage loans held-for-sale.

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- Definition

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.

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