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Carrying amount as of the balance sheet date of the asset arising from derivative instruments and hedging activities, which are expected to be converted into cash or otherwise disposed of within a year or the normal operating cycle, if longer.

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The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

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This item represents the carrying amount on the entity's balance sheet of its investment in common stock of an equity method investee. This is not an indicator of the fair value of the investment, rather it is the initial cost adjusted for the entity's share of earnings and losses of the investee, adjusted for any distributions (dividends) and other than temporary impairment (OTTI) losses recognized.

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Amount of realized gain (loss) from sale of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Amount of investment in equity security without readily determinable fair value.

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Amount of gain from upward price adjustment on investment in equity security without readily determinable fair value.

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Amount of gain (loss) recognized in income from liability measured at fair value on recurring basis using unobservable input (level 3).

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Amount of income (loss) for proportionate share of equity method investee's income (loss).

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Amount of cash inflow from the sale of investments classified as other.

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Fair Value Measurements (Narrative) (Details) $ / shares in Units, shares in Millions, $ in Millions	1 Months Ended		3 Months Ended		12 Months Ended
	Apr. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($)	Dec. 31, 2019 R / shares	Apr. 01, 2019 £ / shares	Dec. 31, 2014 R / shares
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Net increase in estimated fair value of liabilities related to prior period acquisitions					$ 70	$ 32
Increase in fair value of contingent consideration due to 5% increase in projections					25
Decrease in fair value of contingent consideration due to 5% decrease in projections					28
Equity method investments					434	287
Equity method income					12	17
Gains on equity securities						54
Other investments					$ 67	75
Minimum
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Revenue target period					2 years
Maximum
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Revenue target period					4 years
Private Insurance and Consulting
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity method investments					$ 183	61
Private Equity Investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity method investments					107	82
Equity method income					13	16
Equity Investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, fair value					19	146
Other Investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity securities without readily determinable fair value, downward price adjustment, annual amount					$ 1
Equity securities without readily determinable fair value, upward price adjustment, annual amount						1
Money market funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Share price (in dollars per share) \| $ / shares					$ 1.00
JLT Transaction
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Share price (in ZAR per share) \| £ / shares								£ 19.15
Alexander Forbes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Share price (in dollars per share) \| R / shares							R 5.55
Equity method investments					$ 144
Percentage of interest acquired					34.00%				34.00%
Share price (in ZAR per share) \| R / shares									R 7.50
Impairment charge						83
Equity investment shares owned by the Company (in shares) \| shares					443
Market value of equity method investment					$ 173
Benefitfocus
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gains on equity securities					10
Proceeds from sale of other investments	$ 17	$ 132		$ 115
Payscale
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Proceeds from sale of other investments			$ 47
Treasury Lock \| JLT Transaction
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative hedge, asset					2,000	$ 2,000
Fair Value, Measurements, Recurring \| Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Change in contingent purchase consideration					$ 2