Basis of Presentation and Signficant Accounting Policies Basis of Presentation and Significant Acconting Policies (Details) - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Customer funds and cash and cash equivalents on deposit for regulatory requirements	$ 243		$ 182
Premiums Receivable, Allowance for Doubtful Accounts	312		302
Self-funded Receivables, Allowance For Doubtful Accounts	40		$ 153
Adoption of ASU 2018-02	$ 91
Approximate percentage of revenues subject to new revenue guidance under ASC 606	6.00%
Adoption of ASU 2016-01		$ 320

Approximate percentage of revenues subject to new revenue guidance under ASC 606.

No definition available.

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- Definition

Reclassification from accumulated other comprehensive loss to retained earnings for stranded tax effects resulting from the Tax Cuts and Jobs Act.

+ References

No definition available.

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- Definition

The carrying value as of the balance sheet date of the estimated valuation allowance to reduce gross self-funded receivable to net realizable value.

+ References

No definition available.

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- References

No definition available.

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- Definition

Amount after tax of increase (decrease) to equity or (increase) decrease to net assets, resulting from the cumulative effect adjustment of a new accounting principle applied in the period of adoption.

+ References

No definition available.

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- Definition

The carrying value as of the balance sheet date of the estimated valuation allowance to reduce gross premiums receivable to net realizable value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5074-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 4
-Subparagraph (SX 210.12-09)
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24092-122690

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.5)
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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- Definition

The current cash, cash equivalents and investments that are restricted as to withdrawal or usage. Restrictions may include legally restricted deposits held as compensating balances against short-term borrowing arrangements, contracts entered into with others, or entity statements of intention with regard to particular deposits; however, time deposits and short-term certificates of deposit are not generally included in legally restricted deposits. Excludes compensating balance arrangements that are not agreements which legally restrict the use of cash amounts shown on the balance sheet. Includes current cash equivalents and investments that are similarly restricted as to withdrawal, usage or disposal.

+ 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 210.5-02.1)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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