Summary of Significant Accounting Policies - Leases (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Lessee, Lease, Description [Line Items]
Renewal term	20 years
Lease, option to terminate	60 days
Operating lease, liability	us-gaap:OtherLiabilities
Operating lease, right-of-use asset,	us-gaap:OtherAssets
Lease liability	$ 511
Lease, cost	166	$ 171
Variable lease, cost	$ 30	$ 30
Weighted average remaining lease term (years)	5 years	6 years
Weighted average discount rate	3.10%	3.15%
2021	$ 120
2022	116
2023	95
2024	77
2025	63
Thereafter	84
Total lease payments	555
Less: interest	(44)
Present value of lease liabilities	511
Other liabilities & accrued expenses
Lessee, Lease, Description [Line Items]
Lease liability	511	$ 586
Present value of lease liabilities	511	586
Other assets
Lessee, Lease, Description [Line Items]
Right of use asset	$ 393	$ 483
Minimum
Lessee, Lease, Description [Line Items]
Lease, term of contract	1 year
Maximum
Lessee, Lease, Description [Line Items]
Lease, term of contract	9 years

Lessee, Operating Lease, Term of Contract, Option to Terminate

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- Definition

Amount of lease cost recognized by lessee for lease contract.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 50
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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.

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- Definition

Amount of lessee's undiscounted obligation for lease payment for operating lease.

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-Publisher FASB
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-Section 50
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-URI http://asc.fasb.org/extlink&oid=121609121&loc=SL77918701-209980

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- Definition

Amount of lessee's undiscounted obligation for lease payment for operating lease due after fifth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

+ Details

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- Definition

Amount of lessee's undiscounted obligation for lease payment for operating lease to be paid in next fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

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- Definition

Amount of lessee's undiscounted obligation for lease payment for operating lease to be paid in fifth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

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- Definition

Amount of lessee's undiscounted obligation for lease payment for operating lease to be paid in fourth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

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- Definition

Amount of lessee's undiscounted obligation for lease payment for operating lease to be paid in third fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

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- Definition

Amount of lessee's undiscounted obligation for lease payment for operating lease to be paid in second fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

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Period Type:	instant

- Definition

Amount of lessee's undiscounted obligation for lease payments in excess of discounted obligation for lease payments for operating lease.

+ References

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- Definition

Term of lessee's operating lease renewal, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

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-Publisher FASB
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- Definition

Term of lessee's operating lease, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

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- Definition

Present value of lessee's discounted obligation for lease payments from operating lease.

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- Definition

Indicates line item in statement of financial position that includes operating lease liability.

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- Definition

Amount of lessee's right to use underlying asset under operating lease.

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- Definition

Indicates line item in statement of financial position that includes operating lease right-of-use asset.

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- Definition

Weighted average discount rate for operating lease calculated at point in time.

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- Definition

Weighted average remaining lease term for operating lease, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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- Definition

Amount of variable lease cost, excluded from lease liability, recognized when obligation for payment is incurred for finance and operating leases.

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