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Debt (2016 Credit Facility - Narrative) (Details)
1 Months Ended 3 Months Ended 12 Months Ended
Sep. 30, 2017
USD ($)
Jun. 30, 2016
USD ($)
Jan. 31, 2015
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Dec. 31, 2013
USD ($)
Debt Instrument [Line Items]                    
Debt instrument           $ 4,433,000,000 $ 4,433,000,000 $ 4,684,000,000    
Repayments of long-term debt             1,139,000,000 1,175,000,000 $ 500,000,000  
Amount outstanding           0 0      
Long-term Debt [Member]                    
Debt Instrument [Line Items]                    
Maximum borrowing capacity   $ 1,934,000,000                
Debt instrument           4,433,000,000 4,433,000,000 4,643,000,000    
Long-term Debt [Member] | Minimum [Member]                    
Debt Instrument [Line Items]                    
Liquidity requirement           300,000,000 300,000,000      
Long-term Debt [Member] | 2013 Credit Facility [Member]                    
Debt Instrument [Line Items]                    
Maximum borrowing capacity   66,000,000               $ 2,000,000,000
Repayment of line of credit               285,000,000    
Amount outstanding           0 0      
Long-term Debt [Member] | 2016 Credit Facility [Member]                    
Debt Instrument [Line Items]                    
Maximum borrowing capacity   $ 743,000,000                
Long-term Debt [Member] | Letter of Credit [Member]                    
Debt Instrument [Line Items]                    
Amount outstanding           $ 323,000,000 323,000,000      
Revolving Credit Facility And Term Loan Facility [Member] | Long-term Debt [Member]                    
Debt Instrument [Line Items]                    
Maturity date, year and month   2020-12                
Net cash proceeds from asset sales allowed to be retained             $ 500,000,000      
Revolving Credit Facility And Term Loan Facility [Member] | Long-term Debt [Member] | Minimum [Member]                    
Debt Instrument [Line Items]                    
Minimum interest coverage ratio             2.00      
Leverage Ratio             4.00      
Collateral coverage ratio             1.50      
Revolving Credit Facility And Term Loan Facility [Member] | Long-term Debt [Member] | Maximum [Member]                    
Debt Instrument [Line Items]                    
Leverage Ratio           5.50        
Revolving Credit Facility And Term Loan Facility [Member] | Long-term Debt [Member] | Maximum [Member] | Scenario, Forecast [Member]                    
Debt Instrument [Line Items]                    
Leverage Ratio       4.50 5.00          
Revolving Credit Facility And Term Loan Facility [Member] | Long-term Debt [Member] | Eurodollar [Member] | Minimum [Member]                    
Debt Instrument [Line Items]                    
Basis points             1.75%      
Revolving Credit Facility And Term Loan Facility [Member] | Long-term Debt [Member] | Eurodollar [Member] | Maximum [Member]                    
Debt Instrument [Line Items]                    
Basis points             2.50%      
Revolving Credit Facility And Term Loan Facility [Member] | Long-term Debt [Member] | Base Rate [Member] | Minimum [Member]                    
Debt Instrument [Line Items]                    
Basis points             0.75%      
Revolving Credit Facility And Term Loan Facility [Member] | Long-term Debt [Member] | Base Rate [Member] | Maximum [Member]                    
Debt Instrument [Line Items]                    
Basis points             1.50%      
2016 Term Loan due December 2020 [Member] | Long-term Debt [Member]                    
Debt Instrument [Line Items]                    
Maximum borrowing capacity   $ 1,191,000,000                
Debt instrument           $ 1,191,000,000 $ 1,191,000,000 $ 1,191,000,000    
Maturity date, year and month             2020-12 2020-12    
Accelerated maturity date, year and month if not amended, redeemed or refinanced             2019-10      
2016 Term Loan due December 2020 [Member] | Long-term Debt [Member] | Over LIBOR [Member]                    
Debt Instrument [Line Items]                    
Basis points             2.50%      
4.05% Senior Notes due January 2020 [Member] | Long-term Debt [Member]                    
Debt Instrument [Line Items]                    
Debt instrument           $ 92,000,000 $ 92,000,000 $ 850,000,000    
Maturity date, year and month     2020-01       2020-01 2020-01    
Amount to be amended, redeemed or refinanced to avoid acceleration             $ 765,000,000 $ 765,000,000    
Repayments of long-term debt $ 758,000,000           $ 758,000,000