Annual report pursuant to Section 13 and 15(d)

Leases (Details)

v3.8.0.1
Leases (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Operating Leased Assets [Line Items]      
Operating Leases, Rent Expense $ 13 $ 11 $ 10
2018, Minimum Payment [1] 2    
2019, Minimum Payment [1] 2    
2020, Minimum Payment [1] 2    
2021, Minimum Payment [1] 2    
2022, Minimum Payment [1] 2    
Thereafter, Minimum Payment [1] 45    
Total, Minimum Payment [1] $ 55    
Land [Member] | Maximum [Member]      
Operating Leased Assets [Line Items]      
Initial Term of Lease 30 years    
Term of of Available Extension 60 years    
[1] Includes certain lease option renewals that are reasonably assured.