Schedule of Investment Securities Classified as Current and Long-Term (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Investment [Line Items]
Amortized Cost	$ 9,072	$ 9,550
Gross Unrealized Gains	516	377
Gross Unrealized Losses	(41)	(127)
Total debt securities, Fair Value	9,547us-gaap_AvailableForSaleSecurities	9,800us-gaap_AvailableForSaleSecurities
Debt Securities \| Tax-exempt municipal securities
Investment [Line Items]
Amortized Cost	2,931	2,911
Gross Unrealized Gains	140	93
Gross Unrealized Losses	(3)	(33)
Total debt securities, Fair Value	3,068	2,971
Debt Securities \| Residential Mortgage-Backed Securities
Investment [Line Items]
Amortized Cost	17	22
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Total debt securities, Fair Value	17	22
Debt Securities \| Commercial Mortgage-Backed Securities
Investment [Line Items]
Amortized Cost	846	662
Gross Unrealized Gains	16	20
Gross Unrealized Losses	(19)	(9)
Total debt securities, Fair Value	843	673
Debt Securities \| Asset-backed Securities
Investment [Line Items]
Amortized Cost	28	63
Gross Unrealized Gains	1	1
Gross Unrealized Losses	0	(1)
Total debt securities, Fair Value	29	63
Debt Securities \| Corporate Debt Securities
Investment [Line Items]
Amortized Cost	3,432	3,474
Gross Unrealized Gains	299	223
Gross Unrealized Losses	(13)	(30)
Total debt securities, Fair Value	3,718	3,667
Debt Securities \| U.S. Treasury And Other U.S. Government Corporations And Agencies \| U.S. Treasury And Agency Obligations
Investment [Line Items]
Amortized Cost	365	584
Gross Unrealized Gains	10	6
Gross Unrealized Losses	(1)	(6)
Total debt securities, Fair Value	374	584
Debt Securities \| U.S. Treasury And Other U.S. Government Corporations And Agencies \| Mortgage-Backed Securities
Investment [Line Items]
Amortized Cost	1,453	1,834
Gross Unrealized Gains	50	34
Gross Unrealized Losses	(5)	(48)
Total debt securities, Fair Value	$ 1,498	$ 1,820

Name:	invest_InvestmentLineItems
Namespace Prefix:	invest_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 5

-Subparagraph (aa)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562

+ Details

Name:	us-gaap_AvailableForSaleSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ 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 210.5-02.2)

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

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesAmortizedCost
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_DebtSecurityAxis=us-gaap_DebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: