Fair Value of Financial Instruments Fair Value of Financial Instruments (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	$ 0	$ 0	$ 0	$ 0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Transfers, Net	0	$ 0	0	$ 0
Cash Surrender Value, Fair Value Disclosure	28,000,000		28,000,000		$ 28,000,000
Long-term Debt	2,426,000,000		2,426,000,000		2,426,000,000
Unamortized Debt Issuance Expense	10,000,000		10,000,000		10,000,000
Long-term Debt, Fair Value	$ 2,639,000,000		$ 2,639,000,000		$ 2,829,000,000