Fair Value (Nonrecurring Measurements) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset Retirement Obligation	$ 23
Interest Rate Contract [Member] | Measurement Input, Entity Credit Risk [Member] | Weighted Average [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Measurement Inputs, Nonrecurring	4.00%
Interest Rate Contract [Member] | Measurement Input, Entity Credit Risk [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Measurement Inputs, Nonrecurring	4.43%
Interest Rate Contract [Member] | Measurement Input, Entity Credit Risk [Member] | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Measurement Inputs, Nonrecurring	2.00%