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Mortgage Servicing Assets - Changes in Carrying Amount of Mortgage Servicing Assets (Details) - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2021 | Mar. 31, 2020
Commercial Mortgage Backed Securities |  |
Servicing Asset at Amortized Cost, Balance [Roll Forward] |  |
Balance at beginning of period | $ 578 | $ 539
Servicing retained from loan sales | 29 | 24
Purchases | 7 | 11
Amortization | (29) | (29)
Temporary (impairments) recoveries | 3 | (2)
Balance at end of period | 588 | 543
Fair value at end of period | 682 | 655
Agency residential mortgage-backed securities |  |
Servicing Asset at Amortized Cost, Balance [Roll Forward] |  |
Balance at beginning of period | 58 | 46
Servicing retained from loan sales | 11 | 5
Purchases | 0 | 0
Amortization | (5) | (2)
Temporary (impairments) recoveries | 8 | (9)
Balance at end of period | 72 | 40
Fair value at end of period | $ 77 | $ 41

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- Definition Servicing Asset At Amortized Cost, Temporary Impairments (Recoveries) + References No definition available. + Details
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- Definition Amortized amount of contract to service financial assets under which the benefits of servicing are expected to more than adequately compensate the servicer. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 50 -Section 50 -Paragraph 4 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=116651436&loc=d3e122739-111746 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 50 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6469459&loc=d3e122492-111745 Reference 3: http://www.xbrl.org/2003/role/recommendedDisclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 50 -Section 45 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6469459&loc=d3e122501-111745 + Details
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- Definition Amount of additions from purchases, assumption or transfer to contract to service financial assets under which the benefits of servicing are expected to more than adequately compensate the servicer. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 50 -Section 50 -Paragraph 4 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=116651436&loc=d3e122739-111746 + Details
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- Definition Amount of amortization of contract to service financial assets under which the benefits of servicing are expected to more than adequately compensate the servicer. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 50 -Section 50 -Paragraph 4 -Subparagraph (a)(4) -URI http://asc.fasb.org/extlink&oid=116651436&loc=d3e122739-111746 + Details
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- Definition Fair value of contract to service financial assets under which the benefits of servicing are expected to more than adequately compensate the servicer. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 50 -Section 50 -Paragraph 4 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=116651436&loc=d3e122739-111746 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=7516967&loc=d3e66267-113978 + Details
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- Definition Amount of other increase (decrease) to contract to service financial assets under which the benefits of servicing are expected to more than adequately compensate the servicer. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 50 -Section 50 -Paragraph 4 -Subparagraph (a)(7) -URI http://asc.fasb.org/extlink&oid=116651436&loc=d3e122739-111746 + Details
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