Debt - Revolving Credit Facility (Details)	1 Months Ended	6 Months Ended
	Jul. 31, 2017 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
Line of Credit Facility [Line Items]
Debt, Current		$ 548,000,000	$ 686,000,000
Unsecured Debt, Current		990,000,000
Line of Credit Facility, maximum borrowing capacity		$ 3,300,000,000
Ratio of indebtedness to net capital		0.37
Maximum [Member]
Line of Credit Facility [Line Items]
Ratio of indebtedness to net capital		0.65
Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Number of One Year Extensions		2
Line of Credit, Period of Extension Options		1 year
Line of Credit Facility, Remaining Borrowing Capacity		$ 107,000,000
Debentures Due 2017 [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		6.00%
Unsecured Debt, Current		$ 682,000,000
Debentures Due 2018 [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		5.90%
Unsecured Debt, Current		$ 854,000,000
Debentures Due 2019 [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		7.50%
Unsecured Debt, Current		$ 228,000,000
Subsequent Event [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, maximum borrowing capacity	$ 3,400,000,000
Line of Credit Facility, Increase in Borrowing Capacity	93,000,000
Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of Credit, Current		0
Bridge Loan [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, maximum borrowing capacity		100,000,000
Letter of Credit [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, maximum borrowing capacity		$ 500,000,000
Scenario, Forecast [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Gain (Loss) on Extinguishment of Debt	(50,000,000)
Scenario, Forecast [Member] | Minimum [Member]
Line of Credit Facility [Line Items]
Gain (Loss) on Extinguishment of Debt	$ (40,000,000)