Short And Long-Term Debt Maturities Table (Details) - USD ($) $ in Millions		6 Months Ended			12 Months Ended
		Dec. 31, 2018	Jun. 30, 2018		Jun. 30, 2019	Dec. 31, 2017
Maturities of Long-term Debt
Remaining 2018 (a)	[1]		$ 326
2019			480
2020			44
2021			613
2022			$ 81
Long-term Debt Maturities, Years Presented			4 years
Current portion of long-term debt			$ 788			$ 127
Long-term Debt	[2]		3,861
Line of Credit | Revolving Credit Facility
Maturities of Long-term Debt
Line of credit facility outstanding			312	[3],[4]		70
Secured Debt | Term Loan A
Maturities of Long-term Debt
Long-term Debt			740	[5]		390
Secured Debt | Term Loan B
Maturities of Long-term Debt
Long-term Debt			1,058	[6]		1,063
Senior Notes | 4.50% Senior Notes
Maturities of Long-term Debt
Long-term Debt			$ 446			$ 444
Scenario, Forecast | Secured Debt | Term Loan A
Maturities of Long-term Debt
Debt Instrument, Periodic Payment, Principal		$ 9			$ 19
Scenario, Forecast | Secured Debt | Term Loan B
Maturities of Long-term Debt
Debt Instrument, Periodic Payment, Principal		$ 5			$ 11

[1] Remaining 2018 includes amortization payments totaling $9 million and $5 million for the Term Loan A and Term Loan B facilities, respectively, as well as $312 million of revolver borrowings under the Revolving Credit Facility which expires in February 2023, but is classified on the balance sheet as current due to the revolving nature of the facility. The current portion of long term debt of $788 million shown on the condensed consolidated balance sheet consists of $446 million of 4.50% Senior Notes due April 2019, four quarters of amortization payments totaling $19 million and $11 million for the Term Loan A and Term Loan B facilities, respectively, and $312 million of revolver borrowings under the Revolving Credit Facility. The Company is evaluating different alternatives to repay the 4.50% Senior Notes due in April 2019 including refinancing or using existing available liquidity.

[2] Not included in this table is the Company's Unsecured Letter of Credit Facility which had a capacity of $74 million with $65 million utilized at a weighted average rate of 3.24% at June 30, 2018.

[3] As of June 30, 2018, the Company had $1,400 million of borrowing capacity under its Revolving Credit Facility, leaving $1,088 million of available capacity. The Revolving Credit Facility expires in February 2023, but is classified on the balance sheet as current due to the revolving nature of the facility. On August 1, 2018, the Company had $270 million in outstanding borrowings under the Revolving Credit Facility, leaving $1,130 million of available capacity.

[4] Interest rates with respect to revolving loans under the Senior Secured Credit Facility at June 30, 2018 were based on, at the Company's option, (a) adjusted London Interbank Offering Rate ("LIBOR") plus an additional margin or (b) JP Morgan Chase Bank, N.A.'s prime rate ("ABR") plus an additional margin, in each case subject to adjustment based on the then current senior secured leverage ratio. Based on the previous quarter senior secured leverage ratio, the LIBOR margin was 2.25% and the ABR margin was 1.25% for the three months ended June 30, 2018.

[5] The Term Loan A provides for quarterly amortization payments, which commence on June 30, 2018, totaling per annum 2.5%, 2.5%, 5.0%, 7.5% and 10.0% of the original principal amount of the Term Loan A, with the last amortization payment to be made on February 8, 2023. The interest rates with respect to term loans under the Term Loan A are based on, at the Company's option, (a) adjusted LIBOR plus an additional margin or (b) ABR plus an additional margin, in each case subject to adjustment based on the then current senior secured leverage ratio. Based on the previous quarter senior secured leverage ratio, the LIBOR margin was 2.25% and the ABR margin was 1.25% for the three months ended June 30, 2018.

[6] The Term Loan B provides for quarterly amortization payments totaling 1% per annum of the original principal amount. The interest rate with respect to term loans under the Term Loan B is based on, at the Company’s option, (a) adjusted LIBOR plus 2.25% (with a LIBOR floor of 0.75%) or (b) ABR plus 1.25% (with an ABR floor of 1.75%).