Debt Level 4 (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 15, 2018	Dec. 31, 2017
Debt Instruments [Line Items]
Available-for-sale Securities, Equity Securities	$ 0		$ 1,012,000,000
Long-term Debt, Gross		$ 500,000,000
Proceeds from Debt, Net of Issuance Costs	$ 490,000,000
Debt Instrument, Redemption Price, Percentage of Principal Amount Redeemed	100.00%
Debt Instrument, Interest Rate, Increase (Decrease)	2500.00%
Line of Credit Facility, Maximum Borrowing Capacity	$ 250,000,000
Line of Credit Facility, Interest Rate at Period End	0.50%
Line of Credit Facility, Covenant Terms	9
Letters of Credit Outstanding, Amount	$ 3
Line of Credit Facility, Increase (Decrease) for Period, Description	100
Scenario, Previously Reported [Member]
Debt Instruments [Line Items]
Line of Credit Facility, Covenant Terms	13.5
Revolving Credit Facility [Member]
Debt Instruments [Line Items]
Debt Instrument, Term	5 years
Line of Credit Facility, Maximum Borrowing Capacity	$ 750,000,000
Line of Credit Facility, Covenant Terms	0.35
Line of Credit Facility, Maximum Amount Outstanding During Period	$ 0
Revolving Credit Facility [Member] | Scenario, Previously Reported [Member]
Debt Instruments [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 1,000,000,000
Senior Note Six Point Three Due in Two Thousand Eighteen [Member]
Debt Instruments [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		6.30%
Senior Note Four Point Four Due in Two Thousand Forty Seven [Member]
Debt Instruments [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		4.40%
Senior Note Five Point Three Seven Five Due in Two Thousand Seventeen [Member] | Senior Notes
Debt Instruments [Line Items]
Repayment of debt		$ 320,000,000