Summary of Significant Accounting Policies (Narratives) (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) segment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Mar. 31, 2018 USD ($)	Jan. 01, 2018 USD ($)
Schedule of Equity Method Investments [Line Items]
Number of business segments | segment		2
Uncollected premiums and claims, net of related payables		$ 7,300,000,000	$ 6,800,000,000
Restricted cash and cash equivalents		186,000,000
Investment loss		(12,000,000)
Investment income (less than)			15,000,000	$ 1,000,000
Impairment charge	$ 83,000,000
Indefinite-lived intangible assets		0	0
Capitalized computer software costs		435,000,000	488,000,000
Capitalized computer software costs accumulated amortization		1,300,000,000	1,300,000,000
Retained earnings		14,347,000,000	13,140,000,000			$ 13,504,000,000
Effective income tax rate reconciliation, share-based compensation, excess tax benefit, amount		28,000,000	79,000,000
Adjustments to additional paid in capital, income tax benefit from share-based compensation				44,000,000
Unrealized investment (loss) income		0	(7,000,000)	21,000,000
Debt and Equity Securities, Realized Gain (Loss)		$ 17,000,000
Minimum | Software Development
Schedule of Equity Method Investments [Line Items]
Property, plant and equipment, useful life		3 years
Maximum | Software Development
Schedule of Equity Method Investments [Line Items]
Property, plant and equipment, useful life		10 years
Risk and Insurance Services Segment
Schedule of Equity Method Investments [Line Items]
Interest income, fiduciary assets		$ 65,000,000	39,000,000	26,000,000
Consulting Segment
Schedule of Equity Method Investments [Line Items]
Interest income, fiduciary assets		$ 3,000,000	4,000,000	$ 3,000,000
Customer Lists | Minimum
Schedule of Equity Method Investments [Line Items]
Intangible asset useful life		10 years
Customer Lists | Maximum
Schedule of Equity Method Investments [Line Items]
Intangible asset useful life		15 years
Retained Earnings
Schedule of Equity Method Investments [Line Items]
Cumulative effect of amended accounting standard			$ 14,000,000			14,000,000
Accounting Standards Update 2014-09
Schedule of Equity Method Investments [Line Items]
Retained earnings					$ 364,000,000	364,000,000
Accounting Standards Update 2016-16
Schedule of Equity Method Investments [Line Items]
Retained earnings						$ (14,000,000)
Other Liabilities | Minimum
Schedule of Equity Method Investments [Line Items]
Operating lease, liability		$ 200,000,000
Real Estate | Minimum
Schedule of Equity Method Investments [Line Items]
Operating lease, liability		1,800,000,000
Right-of-use asset obtained in exchange for operating lease liability		1,600,000,000
Benefitfocus
Schedule of Equity Method Investments [Line Items]
Unrealized investment (loss) income		$ 54,000,000