Income Taxes - Income Tax Expense (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Current taxes
U.S. income	$ 1,068us-gaap_CurrentFederalTaxExpenseBenefit	$ 382us-gaap_CurrentFederalTaxExpenseBenefit	$ 604us-gaap_CurrentFederalTaxExpenseBenefit
Foreign income	115us-gaap_CurrentForeignTaxExpenseBenefit	77us-gaap_CurrentForeignTaxExpenseBenefit	72us-gaap_CurrentForeignTaxExpenseBenefit
State income	49	42	43
Total taxes, current	1,232us-gaap_CurrentIncomeTaxExpenseBenefit	501us-gaap_CurrentIncomeTaxExpenseBenefit	719us-gaap_CurrentIncomeTaxExpenseBenefit
Deferred taxes (benefits)
U.S. Income	10	152	131
Foreign income	(22)	46	4
State income	(10)	(1)	(1)
Total taxes, deferred	(22)us-gaap_DeferredIncomeTaxExpenseBenefit	197us-gaap_DeferredIncomeTaxExpenseBenefit	134us-gaap_DeferredIncomeTaxExpenseBenefit
Total taxes	$ 1,210us-gaap_IncomeTaxExpenseBenefit	$ 698us-gaap_IncomeTaxExpenseBenefit	$ 853us-gaap_IncomeTaxExpenseBenefit

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