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Fair value, as of the acquisition date, of potential payments under the contingent consideration arrangement including cash and shares.

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Amount of cash paid to acquire the entity.

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Amount of goodwill arising from a business combination, which is the excess of the cost of the acquired entity over the amounts assigned to assets acquired and liabilities assumed.

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Amount of acquisition cost of a business combination allocated to assets, excluding financial assets and goodwill, lacking physical substance.

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The amount of acquisition cost of a business combination allocated to property, plant and equipment to be used in ongoing operations.

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Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The number of grants made 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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Weighted average remaining contractual term for equity-based awards excluding options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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- Definition

The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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The risk-free interest rate assumption that is used in valuing an option on its own shares.

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Stock-Based Compensation (Narrative) (Detail) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Business Acquisition, Purchase Price Allocation, Intangible Assets Other than Goodwill	$ 188		$ 188
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate			43.00%	50.00%
Risk Free Interest Rate, Minimum			0.39%
Risk Free Interest Rate, Maximum				0.42%
Valuation Period Range Maximum In Years			3 years	2 1/2 - 3 years
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Rate			0.00%	0.00%
Stock-based compensation expense	5	4	14	12
Stock-based compensation expense (net of tax)	3	2	9	7
Employee service share based compensation incremental tax benefit to be realized from exercise of stock awards	13		13
Restricted Cash Units Outstanding	268,000		268,000
Customer Relationships	76		76
Time-based vesting schedule [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of restricted stock units which contain three-year vesting periods			$ 21.77
Market-based vesting schedule [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of restricted stock units which contain three-year vesting periods	$ 17.14			$ 10.92
Performance-Based Vesting Schedule [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of restricted stock units which contain three-year vesting periods	$ 20.67
Restricted Cash Units (RCUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average remaining contractual life (in years)	1 year 7 months 24 days
Restricted Cash Units (RCUs) [Member] \| Time-based vesting schedule [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted at fair market value (in number of RSUs)			25,000
Fair Value of Time-Based Restricted Cash Units			$ 21.20
Restricted Cash Units (RCUs) [Member] \| Market-based vesting schedule [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted at fair market value (in number of RSUs)			87,000
Fair Value of Market-Vesting Restricted Cash Units			$ 17.14
Restricted Stock Units (RSUs) [Member] \| Time-based vesting schedule [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted at fair market value (in number of RSUs)			637,000
Restricted Stock Units (RSUs) [Member] \| Market-based vesting schedule [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted at fair market value (in number of RSUs)			87,000
Restricted Stock Units (RSUs) [Member] \| Performance-Based Vesting Schedule [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted at fair market value (in number of RSUs)			396,000
Zipcar [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Business Acquisition, Purchase Price Allocation, Intangible Assets Other than Goodwill	112		112
Business Acquisition, Purchase Price Allocation, Goodwill Amount	275		275
Finite-Lived Intangible Asset, Useful Life			8 years
Business Acquisition, Effective Date of Acquisition	Mar. 14, 2013
Apex [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Business Acquisition, Purchase Price Allocation, Intangible Assets Other than Goodwill	21		21
Business Acquisition, Purchase Price Allocation, Goodwill Amount	16		16
Business Acquisition, Effective Date of Acquisition	Oct. 01, 2012
Number Of Vehicles	4,000		4,000
Business Acquisition, Cost of Acquired Entity, Cash Paid	63		63
Business Acquisition, Contingent Consideration, at Fair Value	9		9
Business Acquisition, Contingent Consideration, Potential Cash Payment	26		26
Business Acquisition, Purchase Price Allocation, Property, Plant and Equipment	33		33
Business Acquisition, Contingent Consideration, Potential Cash Payment	26		26
Business Acquisition, Purchase Price Allocation, Property, Plant and Equipment	$ 33		$ 33