Mortgage Servicing Assets - Schedule of Range and Weighted-Average of Significant Unobservable Inputs (Details) - Discounted Cash Flow [Member] - USD ($)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Commercial Mortgage Backed Securities [Member] \| Minimum [Member]
Servicing Assets at Fair Value [Line Items]
Expected defaults/Discount rate (as a percent)	1.00%	1.00%
Residual cash flows discount rate (as a percent)	7.00%	7.00%
Escrow earn rate (as a percent)	0.90%	1.10%
Servicing cost	$ 150,000,000	$ 150,000,000
Loan assumption rate (as a percent)	0.00%	0.00%
Percentage late (as a percent)	0.00%	0.00%
Commercial Mortgage Backed Securities [Member] \| Maximum [Member]
Servicing Assets at Fair Value [Line Items]
Expected defaults/Discount rate (as a percent)	3.00%	3.00%
Residual cash flows discount rate (as a percent)	15.00%	12.00%
Escrow earn rate (as a percent)	3.10%	3.00%
Servicing cost	$ 38,500,000,000	$ 2,700,000,000
Loan assumption rate (as a percent)	3.00%	3.00%
Percentage late (as a percent)	1.30%	2.00%
Commercial Mortgage Backed Securities [Member] \| Weighted Average [Member]
Servicing Assets at Fair Value [Line Items]
Expected defaults/Discount rate (as a percent)	1.20%	1.40%
Residual cash flows discount rate (as a percent)	9.10%	8.00%
Escrow earn rate (as a percent)	2.50%	2.40%
Servicing cost	$ 1,468,000,000	$ 1,214,000,000
Loan assumption rate (as a percent)	1.22%	1.13%
Percentage late (as a percent)	0.25%	0.34%
Residential Mortgage Backed Securities [Member] \| Minimum [Member]
Servicing Assets at Fair Value [Line Items]
Expected defaults/Discount rate (as a percent)	8.50%	8.50%
Servicing cost	$ 76,000,000	$ 76,000,000
Prepayment speed (as a percent)	9.16%	7.79%
Residential Mortgage Backed Securities [Member] \| Maximum [Member]
Servicing Assets at Fair Value [Line Items]
Expected defaults/Discount rate (as a percent)	11.00%	11.00%
Servicing cost	$ 4,385,000,000	$ 3,335,000,000
Prepayment speed (as a percent)	51.52%	18.61%
Residential Mortgage Backed Securities [Member] \| Weighted Average [Member]
Servicing Assets at Fair Value [Line Items]
Expected defaults/Discount rate (as a percent)	8.54%	8.55%
Servicing cost	$ 83,110,000	$ 830,400
Prepayment speed (as a percent)	10.46%	9.42%

Interest rate used to account for mortgage loan at fair value upon assumption.

No definition available.

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- Definition

Mortgage servicing assets, assumptions used to estimate fair value, percentage late.

+ References

No definition available.

+ Details

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- Definition

Mortgage servicing assets interest rate for mortgage funds.

+ References

No definition available.

+ Details

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- Definition

Mortgage servicing assets servicing cost.

+ References

No definition available.

+ Details

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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

Percentage of contractually collectible principal associated with a financial asset expected not to be collected due to default by the borrower.

+ References

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- Definition

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

+ References

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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.

+ References

No definition available.

+ Details

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