Business and Basis of Presentation (Narrative) (Details) - USD ($)	1 Months Ended	3 Months Ended								12 Months Ended			36 Months Ended
	Oct. 31, 2019	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2022	Oct. 01, 2019	Sep. 04, 2019
Business Acquisition [Line Items]
Net income (loss)		$ 1,138,000,000	$ 1,425,000,000	$ 738,000,000	$ 937,000,000	$ 849,000,000	$ 1,675,000,000	$ 200,000,000	$ 1,364,000,000	$ 4,238,000,000	$ 4,088,000,000	$ 7,974,000,000
Share price (usd per share)														$ 87.67	$ 83.71
Intangible assets acquired		191,000,000
Goodwill, acquired during period		2,128,000,000								2,150,000,000	$ 22,000,000
Acquisition of Assurance IQ, Inc.
Business Acquisition [Line Items]
Consideration transferred	$ 2,212,000,000
Contingent consideration, liability	100,000,000	105,000,000								105,000,000
Transaction costs	1,758,000,000
Equity interest issued	454,000,000
Equity interest issued, value	$ 160,000,000
Cash | Acquisition of Assurance IQ, Inc.
Business Acquisition [Line Items]
Equity interest in acquiree	25.00%
Common Stock | Acquisition of Assurance IQ, Inc.
Business Acquisition [Line Items]
Equity interest in acquiree	75.00%
Condition One | Subsequent Event | Acquisition of Assurance IQ, Inc.
Business Acquisition [Line Items]
Net income (loss)													$ 900,000,000
Condition Three | Acquisition of Assurance IQ, Inc.
Business Acquisition [Line Items]
Contingent liability, stock based compensation component		$ 900,000,000								$ 900,000,000
Share price (usd per share)															$ 83.71
Condition Four - Quotient - Numerator | Subsequent Event | Acquisition of Assurance IQ, Inc.
Business Acquisition [Line Items]
Net income (loss)													900,000,000
Condition Four - Quotient - Denominator | Acquisition of Assurance IQ, Inc.
Business Acquisition [Line Items]
Consideration payable, cash and equity interests	$ 400,000,000
Condition Four - Multiplier | Acquisition of Assurance IQ, Inc.
Business Acquisition [Line Items]
Consideration payable, cash and equity interests	1,150,000,000
Minimum | Condition One | Acquisition of Assurance IQ, Inc.
Business Acquisition [Line Items]
Consideration payable, cash and equity interests	0
Minimum | Condition Two | Subsequent Event | Acquisition of Assurance IQ, Inc.
Business Acquisition [Line Items]
Net income (loss)													1,300,000,000
Minimum | Condition Three | Subsequent Event | Acquisition of Assurance IQ, Inc.
Business Acquisition [Line Items]
Net income (loss)													900,000,000
Maximum | Condition Two | Acquisition of Assurance IQ, Inc.
Business Acquisition [Line Items]
Consideration payable, cash and equity interests	$ 1,150,000,000
Maximum | Condition Three | Subsequent Event | Acquisition of Assurance IQ, Inc.
Business Acquisition [Line Items]
Net income (loss)													$ 1,300,000,000