Leases (Details) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
		Mar. 31, 2019	Dec. 31, 2018
Lessee, Lease, Description [Line Items]
Operating lease expense		$ 9
Variable lease payments not included in the measurement of lease liabilities		4
Short-term lease payments not included in the measurement of lease liabilities		3
Cash paid for amounts included in the measurement of lease liabilities		9
Lease obligations obtained in exchange for right-of-use assets (non-cash)		176
Operating lease right of use assets		158
Operating lease liabilities - current		33
Operating lease liabilities - noncurrent		134
Total operating lease liabilities		$ 167
Weighted-average remaining lease term—operating leases (years)		6 years 8 months 8 days
Weighted-average discount rate—operating leases		3.69%
2019	[1]	$ 28
2020		34
2021		28
2022		24
2023		19
After 2023		59
Total Lease Payments		192
Less: Imputed Interest		$ (25)
2019			$ 38
2020			30
2021			24
2022			21
2023			15
After 2023			45
Total			173
Total rent expense			$ 45
Minimum
Lessee, Lease, Description [Line Items]
Weighted-average remaining lease term—operating leases (years)		1 year
Maximum
Lessee, Lease, Description [Line Items]
Weighted-average remaining lease term—operating leases (years)		15 years

[1] 2019 excludes the three months ended March 31, 2019