Short And Long-Term Debt Senior Secured Credit Facility (Details) - USD ($) $ in Millions		6 Months Ended
		Jun. 30, 2018		Feb. 28, 2018	Dec. 31, 2017	Jan. 31, 2017	Jul. 20, 2016	Mar. 05, 2013
Debt Instrument [Line Items]
Long-term debt principal amount	[1]	$ 3,893
Letter of Credit, borrowing capacity		$ 125
Additional Credit Facilities								$ 500
Maximum | Required Covenant Ratio to Receive Additional Credit Facilities
Debt Instrument [Line Items]
Senior secured leverage ratio		3.50
Ratio of Indebtedness to Net Capital Denominator		1.00
Maximum | Required Covenant Ratio
Debt Instrument [Line Items]
Senior secured leverage ratio		4.75
Ratio of Indebtedness to Net Capital Denominator		1.00
LIBOR
Debt Instrument [Line Items]
Description of variable interest rate basis		LIBOR
ABR
Debt Instrument [Line Items]
Description of variable interest rate basis		ABR
Term Loan B | LIBOR
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate		2.25%
Debt Instrument, Basis Spread on Variable Rate, Floor		0.75%
Term Loan B | ABR
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate		1.25%
Debt Instrument, Basis Spread on Variable Rate, Floor		1.75%
Line of Credit | Revolving Credit Facility
Debt Instrument [Line Items]
Line of credit facility borrowing capacity		$ 1,400	[2],[3]	$ 1,400		$ 1,050
Line of Credit | Revolving Credit Facility | LIBOR | Greater than 3.50 to 1.00
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate		2.50%
Line of Credit | Revolving Credit Facility | LIBOR | Less than or equal to 3.50 to 1.00 but greater than or equal to 2.50 to 1.00
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate		2.25%
Line of Credit | Revolving Credit Facility | LIBOR | Less than 2.50 to 1.00 but greater than or equal to 2.00 to 1.00
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate		2.00%
Line of Credit | Revolving Credit Facility | LIBOR | Less than 2.00 to 1.00
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate		1.75%
Line of Credit | Revolving Credit Facility | ABR | Greater than 3.50 to 1.00
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate		1.50%
Line of Credit | Revolving Credit Facility | ABR | Less than or equal to 3.50 to 1.00 but greater than or equal to 2.50 to 1.00
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate		1.25%
Line of Credit | Revolving Credit Facility | ABR | Less than 2.50 to 1.00 but greater than or equal to 2.00 to 1.00
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate		1.00%
Line of Credit | Revolving Credit Facility | ABR | Less than 2.00 to 1.00
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate		0.75%
Secured Debt | Term Loan B
Debt Instrument [Line Items]
Repurchased amount of debt							$ 1,100
Long-term debt principal amount		$ 1,075	[4]	$ 1,080	$ 1,083
Annual percentage of original principal amount for quarterly amortization payments		1.00%

[1] 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.

[2] 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.

[3] 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.

[4] 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%).