Fair Value - Significant unobservable inputs (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2017	Dec. 31, 2016
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Carrying Value	$ 62,706,268	$ 77,256,266
Carrying Value	58,166,534	$ 72,696,193
Fair value measurements on recurring basis \| Level III \| VIE liabilities \| Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Carrying Value	$ 2,259,783
Fair value measurements on recurring basis \| Level III \| VIE liabilities \| Discounted cash flow \| Minimum
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Yield (as a percent)	0.00%	0.00%
Duration	0 years	0 years
Fair value measurements on recurring basis \| Level III \| VIE liabilities \| Discounted cash flow \| Maximum
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Yield (as a percent)	813.90%	960.40%
Duration	11 years 7 months 6 days	12 years
Fair value measurements on recurring basis \| Level III \| Loans held-for-sale \| Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Carrying Value	$ 608,624
Duration		10 years
Fair value measurements on recurring basis \| Level III \| Loans held-for-sale \| Discounted cash flow \| Minimum
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Yield (as a percent)	4.30%	5.00%
Duration	3 years 3 months 18 days
Fair value measurements on recurring basis \| Level III \| Loans held-for-sale \| Discounted cash flow \| Maximum
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Yield (as a percent)	5.90%	5.70%
Duration	12 years 9 months 18 days
Fair value measurements on recurring basis \| Level III \| RMBS \| Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Carrying Value	$ 253,252
Portfolio percentage	81.00%	57.00%
Fair value measurements on recurring basis \| Level III \| RMBS \| Discounted cash flow \| Minimum
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Constant prepayment rate (as a percent)	2.40%	2.80%
Constant default rate (as a percent)	0.80%	1.10%
Loss severity (as a percent)	16.00%	12.00%
Delinquency rate (as a percent)	4.00%	2.00%
Servicer advances (as a percent)	20.00%	23.00%
Annual coupon deterioration (as a percent)	0.00%	0.00%
Putback amount per projected total collateral loss (as a percent)	0.00%	0.00%
Loss severity for specified percentage of portfolio (as a percent)	45.00%	45.00%
Fair value measurements on recurring basis \| Level III \| RMBS \| Discounted cash flow \| Maximum
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Constant prepayment rate (as a percent)	19.40%	17.00%
Constant default rate (as a percent)	5.80%	8.10%
Loss severity (as a percent)	79.00%	79.00%
Delinquency rate (as a percent)	34.00%	29.00%
Servicer advances (as a percent)	84.00%	94.00%
Annual coupon deterioration (as a percent)	0.80%	0.60%
Putback amount per projected total collateral loss (as a percent)	15.00%	15.00%
Loss severity for specified percentage of portfolio (as a percent)	80.00%	80.00%
Fair value measurements on recurring basis \| Level III \| CMBS \| Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Carrying Value	$ 23,841
Fair value measurements on recurring basis \| Level III \| CMBS \| Discounted cash flow \| Minimum
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Yield (as a percent)	0.00%	0.00%
Duration	0 years	0 years
Fair value measurements on recurring basis \| Level III \| CMBS \| Discounted cash flow \| Maximum
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Yield (as a percent)	167.10%	172.00%
Duration	9 years 10 months 24 days	18 years 8 months 12 days
Fair value measurements on recurring basis \| Level III \| Domestic servicing rights \| Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Carrying Value	$ 33,781
Yield (as a percent)	7.75%	7.75%
Discount rate (as a percent)	15.00%	15.00%
Fair value measurements on recurring basis \| Level III \| Domestic servicing rights \| Discounted cash flow \| Minimum
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Control migration (as a percent)	0.00%	0.00%
Fair value measurements on recurring basis \| Level III \| Domestic servicing rights \| Discounted cash flow \| Maximum
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Control migration (as a percent)	80.00%	80.00%
Fair value measurements on recurring basis \| Level III \| VIE Assets \| Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Carrying Value	$ 51,197,981
Fair value measurements on recurring basis \| Level III \| VIE Assets \| Discounted cash flow \| Minimum
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Yield (as a percent)	0.00%	0.00%
Duration	0 years	0 years
Fair value measurements on recurring basis \| Level III \| VIE Assets \| Discounted cash flow \| Maximum
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Yield (as a percent)	813.90%	960.40%
Duration	11 years 7 months 6 days	12 years

Represents the annual coupon deterioration, used as an input to measure fair value.

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Represents the constant default rate, used as an input to measure fair value.

+ References

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Represents the constant prepayment rate, used as an input to measure fair value.

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- Definition

Control migration of financial instrument used as an input to measure fair value.

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- Definition

Represents the delinquency rate, used as an input to measure fair value.

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Duration of financial instrument used as an input to measure fair value.

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- Definition

Loss, including interest and principal, incurred on a defaulted security, expressed as a percentage of the original principal balance, used as an input to measure fair value for a specified percentage of the portfolio.

+ References

No definition available.

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- Definition

Represents the putback amount per projected total collateral loss, used as an input to measure fair value.

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- Definition

Represents the servicer advances, used as an input to measure fair value.

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- Definition

Represents the specified percentage of the portfolio falling within a narrower range of loss severity assumptions used as an input to measure fair value.

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- Definition

Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are 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 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(12))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(11))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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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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- Definition

Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.

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

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Percentage of assumed long-term growth in revenues, used as an input to measure fair value.

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- Definition

Loss, including interest and principal, incurred on a defaulted security, expressed as a percentage of the original principal balance, used as an input to measure fair value.

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- Definition

Sum of the carrying amounts as of the balance sheet date of all liabilities that are recognized. Liabilities are probable future sacrifices of economic benefits arising from present obligations of an entity to transfer assets or provide services to other entities in the future.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19-26)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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