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Subsequent Events (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended				12 Months Ended
	Feb. 11, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2012
Subsequent Event [Line Items]
Debt Instrument, Issuance Date							Oct. 01, 2012
Debt Instrument, Face Amount							$ 1,650
Debt Instrument, Interest Rate, Stated Percentage							5.25%
Debt Instrument Maturity Date Range							Jan. 01, 2023
Common Stock, Dividends, Per Share, Cash Paid		$ 1.125	$ 1.05	$ 1.05	$ 1.05
Dividend Declared [Member] \| Common Stock [Member]
Subsequent Event [Line Items]
Dividends Payable, Date Declared						Feb. 21, 2019
Dividends Payable, Date to be Paid						Mar. 29, 2019
Dividends Payable, Date of Record						Mar. 15, 2019
Paid subsequent to year end [Member]
Subsequent Event [Line Items]
Common Stock, Dividends, Per Share, Cash Paid		$ 1.125
Debt [Member] \| February 2019 Senior Notes [Domain] \| High Yield Bonds [Member]
Subsequent Event [Line Items]
Debt Instrument, Issuance Date	Feb. 11, 2019
Debt Instrument, Face Amount	$ 1,000
Debt [Member] \| February 2019 Senior Unsecured 4.300% Notes [Member] \| High Yield Bonds [Member]
Subsequent Event [Line Items]
Debt Instrument, Face Amount	$ 600
Debt Instrument, Interest Rate, Stated Percentage	4.30%
Debt Instrument Maturity Date Range	Feb. 01, 2029
Debt [Member] \| February 2019 Senior Unsecured 5.200% Notes [Member] \| High Yield Bonds [Member]
Subsequent Event [Line Items]
Debt Instrument, Face Amount	$ 400
Debt Instrument, Interest Rate, Stated Percentage	5.20%
Debt Instrument Maturity Date Range	Feb. 01, 2049