Fair Value of Financial Instruments Measured at Fair Value - Additional Information about Loans, Notes and Certificates Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Loans, Fair value, Beginning Balance	$ 2,798,505	$ 1,829,042
Loans, Purchases of loans	1,474,972	776,801
Loans, Issuances of notes and certificates	0	0
Loans, Principal payments	(622,145)	(265,582)
Loans, Whole loan sales	(369,379)	(204,950)
Loans, Recoveries from sale and collection of charged-off loans	(3,472)	(744)
Loans, Carrying value before fair value adjustments	3,278,481	2,134,567
Loans, Fair value adjustments, included in earnings	(47,820)	(24,749)
Loans, Fair value, Ending Balance	3,230,661	2,109,818
Notes and Certificates, Fair value, Beginning Balance	2,813,618	1,839,990
Notes and Certificates, Purchases of loans	0	0
Notes and Certificates, Issuances of notes and certificates	852,715	510,962
Notes and Certificates, Principal payments	0	0
Notes and Certificates, Whole loan sales	(365,711)	(205,274)
Notes and Certificates, Recoveries and sale of charged-off loans	(3,461)	(734)
Notes and Certificates, Carrying value before fair value adjustments	3,297,161	2,144,944
Notes and Certificates, Fair value adjustments, included in earnings	(47,815)	(24,717)
Notes and Certificates, Fair value, Ending Balance	$ 3,249,346	$ 2,120,227

Fair Value of Financial Instruments Measured at Fair Value - Additional Information about Loans, Notes and Certificates Measured at Fair Value on Recurring Basis (Detail) (USD $)
In Thousands, unless otherwise specified

3 Months Ended

Mar. 31, 2015

Mar. 31, 2014

Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]

Loans, Fair value, Beginning Balance

$ 2,798,505us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue

$ 1,829,042us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue

Loans, Purchases of loans

1,474,972us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases

776,801us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases

Loans, Issuances of notes and certificates

0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssues

Loans, Principal payments

(622,145)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements

(265,582)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements

Loans, Whole loan sales

(369,379)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales

(204,950)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales

Loans, Recoveries from sale and collection of charged-off loans

(3,472)ck0001409970_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetRecoveriesFromSaleAndCollectionOfChargedOffLoans

(744)ck0001409970_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetRecoveriesFromSaleAndCollectionOfChargedOffLoans

Loans, Carrying value before fair value adjustments

3,278,481ck0001409970_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPrincipalAmountOfLoanBeforeFairValueAdjustment

2,134,567ck0001409970_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPrincipalAmountOfLoanBeforeFairValueAdjustment

Loans, Fair value adjustments, included in earnings

(47,820)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1

(24,749)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1

Loans, Fair value, Ending Balance

3,230,661us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue

2,109,818us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue

Notes and Certificates, Fair value, Beginning Balance

2,813,618us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue

1,839,990us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue

Notes and Certificates, Purchases of loans

0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases

Notes and Certificates, Issuances of notes and certificates

852,715us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityIssues

510,962us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityIssues

Notes and Certificates, Principal payments

0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements

Notes and Certificates, Whole loan sales

(365,711)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySales

(205,274)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySales

Notes and Certificates, Recoveries and sale of charged-off loans

(3,461)ck0001409970_RecoveriesFromSaleAndCollectionOfChargedOffLoansForNotesAndCertificates

(734)ck0001409970_RecoveriesFromSaleAndCollectionOfChargedOffLoansForNotesAndCertificates

Notes and Certificates, Carrying value before fair value adjustments

3,297,161ck0001409970_PrincipalAmountOfNotesBeforeFairValueAdjustment

2,144,944ck0001409970_PrincipalAmountOfNotesBeforeFairValueAdjustment

Notes and Certificates, Fair value adjustments, included in earnings

(47,815)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings

(24,717)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings

Notes and Certificates, Fair value, Ending Balance

$ 3,249,346us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue

$ 2,120,227us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue

- Definition

Fair Value Measurement with Unobservable Inputs Reconciliation Recurring Basis Asset Principal Amount of Loan before Fair Value Adjustment

+ References

No definition available.

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- Definition

Fair Value Measurement with Unobservable Inputs Reconciliation Recurring Basis Asset Recoveries from Sale and Collection of Charged Off Loans

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No definition available.

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Recoveries from Sale and Collection of Charged Off Loans for Notes and Certificates

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No definition available.

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- Definition

Amount of gain (loss) recognized in the income statement for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

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-Section 50

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Amount of issuances of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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-Publisher FASB

-Name Accounting Standards Codification

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Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

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Amount of sale of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

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Amount of settlement of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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-Name Accounting Standards Codification

-Topic 820

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Amount of gain (loss) recognized on the income statement for financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of sales of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of settlements of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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