Employee Benefit Plans - Additional Information (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee retirement and savings plan cost	$ 176	$ 155	$ 138
Closing stock price as of December 31, 2014	$ 143.63hum_ClosingStockPrice
Percentage of retirement and savings plan's assets invested in our company common stock	13.00%
Shares of retirement and savings plans invested in company's common stock	2.8
Percentage of the retirement and savings plan's assets invested in our common stock as compared to company's outstanding shares at year-end	2.00%
Number of shares of our common stock reserved for issuance under our defined contribution retirement savings plans	4.1
Combination of age and years necessary for retirement provision as related to our equity award program	65 years
Minimum age required for retirement eligibility as related to our equity award program	55 years
Minimum number of years of service for retirement eligibility as related to our equity award program	5 years
Employee service share based compensation tax benefit realized from exercise of stock options and restricted stock vesting	30	20	43
Common stock shares reserved for stock award plans	19.3
Fair value of shares vested	99	52	75
Outstanding intrinsic value of stock options	49
Weighted-average remaining contractual term, years	4 years 4 months 24 days
Exercisable stock options intrinsic value	14
Exercisable stock options weighted-average remaining contractual term, years	3 years 1 month 6 days
Total intrinsic value of stock options exercised	32	39	45
Cash received from stock options exercises	52	67	60
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average grant date fair value of restricted stock awards	$ 103.57	$ 73.50	$ 85.29
Award vesting period	3 years
Total compensation expense not yet recognized related to nonvested restricted stock	92
Compensation expense over a weighted-average period, years	1 year 8 months 12 days
Employee Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total compensation expense not yet recognized related to nonvested restricted stock	$ 6
Compensation expense over a weighted-average period, years	1 year 7 months 6 days
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of year for stock options to vest	1 year
Number of years for stock options to expire	7 years
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of year for stock options to vest	3 years
Number of years for stock options to expire	10 years
2011 Plan [Member] \| Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock shares available for future grants if all grants are options	4.6
2011 Plan [Member] \| Employee Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock shares available for future grants if all grants are options	10.6

Closing stock price for the company per the New York Stock Exchange

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Combination of age and years necessary for retirement provision as related to our equity award program

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Disclosure of the aggregate tax benefit realized from the exercise of stock options and vesting of restricted stock

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Minimum age required for retirement eligibility as related to our equity award program

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Minimum number of years of service for retirement eligibility as related to our equity award program

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Number of shares of our common stock reserved for issuance under our defined contribution retirement savings plans

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Number of year for stock options to vest

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Percentage of the retirement and savings plan's assets invested in our common stock as compared to company's outstanding shares at year-end

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Percentage of retirement and savings plan's assets invested in our company common stock

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Number of common shares available for grant assuming all options

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Shares Of Retirement And Savings Plans Invested In Our Companys Common Stock

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The amount of the cost recognized during the period for defined contribution plans.

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Aggregate proceeds received by the entity during the annual period from exercises of stock or unit options and conversion of similar instruments granted under equity-based payment arrangements.

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Unrecognized cost of unvested share-based compensation awards.

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Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash.

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Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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