Debt (First Lien Term Loans) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Long-term Debt	$ 11,368	$ 11,213
New 2019 First Lien Term Loan [Member]
Debt Instrument [Line Items]
Long-term Debt	389	389
2023 First Lien Term Loan [Member]
Debt Instrument [Line Items]
Long-term Debt	1,063	1,064
2024 First Lien Term Loan [Member]
Debt Instrument [Line Items]
Long-term Debt	1,538	1,542
First Lien Term Loans [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 2,990	$ 2,995