Debt - Revolving Credit Facility (Details)	9 Months Ended
	Sep. 30, 2017 USD ($)	Jul. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Line of Credit Facility [Line Items]
Debt, Current	$ 0		$ 686,000,000
Line of Credit Facility, maximum borrowing capacity	$ 3,400,000,000	$ 3,300,000,000
Line of Credit Facility, Increase in Borrowing Capacity		$ 93,000,000
Ratio of indebtedness to net capital	0.36
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
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