Income Taxes - Rollforward of Gross Consolidated Liability for Unrecognized Income Tax Benefits (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits, beginning of year	$ 266us-gaap_UnrecognizedTaxBenefits	$ 270us-gaap_UnrecognizedTaxBenefits	$ 197us-gaap_UnrecognizedTaxBenefits
Tax positions related to current year:
Additions	63	37	25
Reductions	0	0	(1)
Tax positions related to prior years:
Additions	91	10	64
Reductions	(31)	(33)	(4)
Settlements	(1)	(5)	0
Lapses in statutes of limitations	(5)	(13)	(11)
Unrecognized tax benefits, end of year	$ 383us-gaap_UnrecognizedTaxBenefits	$ 266us-gaap_UnrecognizedTaxBenefits	$ 270us-gaap_UnrecognizedTaxBenefits

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Amount of unrecognized tax benefits pertaining to uncertain tax positions taken in tax returns.

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Amount of decrease in unrecognized tax benefits resulting from tax positions that have been or will be taken in current period tax return.

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Amount of decrease in unrecognized tax benefits resulting from tax positions taken in prior period tax returns.

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Amount of decrease in unrecognized tax benefits resulting from settlements with taxing authorities.

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Amount of increase in unrecognized tax benefits resulting from tax positions that have been or will be taken in current period tax return.

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Amount of decrease in unrecognized tax benefits resulting from lapses of applicable statutes of limitations.

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