Mortgage Servicing Assets - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Commercial Mortgage Backed Securities [Member]
Servicing Assets at Fair Value [Line Items]
Contractual fee income	$ 171	$ 150	$ 139
Amortization of mortgage servicing rights	102	$ 90	$ 87
Residential Mortgage Backed Securities [Member]
Servicing Assets at Fair Value [Line Items]
Contractual fee income	14
Amortization of mortgage servicing rights	$ 4

The periodic amortization (in proportion to and over the period of estimated net servicing income or loss) of capitalized servicing rights, which contractually entitle the servicer to receive fees and ancillary revenues for performing billing, collection, disbursement and recordkeeping services in connection with a mortgage portfolio. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 50
-Paragraph 4
-Subparagraph (a)(4)
-URI http://asc.fasb.org/extlink&oid=68078379&loc=d3e122739-111746

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