Derivative Financial Instruments (Summary Of Outstanding Fair Value Hedges) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Derivative [Line Items]
Contractual Notional Amount	$ 1,653	$ 2,048
Interest Rate Swaps [Member]
Derivative [Line Items]
Contractual Notional Amount	1,200	1,235
Interest Rate Swaps [Member] \| 2018 [Member] \| Interest Rate Received Four Point One Zero One Percentage [Member]
Derivative [Line Items]
Contractual Notional Amount	$ 50	0
Derivative, Fixed Interest Rate	4.101%
Investment Options, Expiration Date	Sep. 01, 2027
Interest Rate Swaps [Member] \| 2018 [Member] \| Interest Rate Received 3.300% [Member]
Derivative [Line Items]
Contractual Notional Amount	$ 450	0
Derivative, Fixed Interest Rate	3.30%
Investment Options, Expiration Date	Jan. 15, 2023
Interest Rate Swaps [Member] \| 2018 [Member] \| Interest Rate Received 4.350% [Member]
Derivative [Line Items]
Contractual Notional Amount	$ 90	0
Derivative, Fixed Interest Rate	4.35%
Investment Options, Expiration Date	Aug. 15, 2020
Interest Rate Swaps [Member] \| 2017 [Member] \| Interest Rate Received 4.350% [Member]
Derivative [Line Items]
Contractual Notional Amount	$ 50	$ 50
Derivative, Fixed Interest Rate	4.35%	4.35%
Investment Options, Expiration Date	Aug. 15, 2020	Aug. 15, 2020
Interest Rate Swaps [Member] \| 2015 [Member] \| Interest Rate Received 4.350% [Member]
Derivative [Line Items]
Contractual Notional Amount	$ 200	$ 200
Derivative, Fixed Interest Rate	4.35%	4.35%
Investment Options, Expiration Date	Aug. 15, 2020	Aug. 15, 2020
Interest Rate Swaps [Member] \| 2014 [Member] \| Interest Rate Received 4.350% [Member]
Derivative [Line Items]
Contractual Notional Amount	$ 150	$ 150
Derivative, Fixed Interest Rate	4.35%	4.35%
Investment Options, Expiration Date	Aug. 15, 2020	Aug. 15, 2020
Interest Rate Swaps [Member] \| 2013 [Member] \| Interest Rate Received 4.350% [Member]
Derivative [Line Items]
Contractual Notional Amount	$ 10	$ 10
Derivative, Fixed Interest Rate	4.35%	4.35%
Investment Options, Expiration Date	Aug. 15, 2020	Aug. 15, 2020
Interest Rate Swaps [Member] \| 2012 [Member] \| Interest Rate Received 4.350% [Member]
Derivative [Line Items]
Contractual Notional Amount	$ 200	$ 200
Derivative, Fixed Interest Rate	4.35%	4.35%
Investment Options, Expiration Date	Aug. 15, 2020	Aug. 15, 2020
Interest Rate Swaps [Member] \| 2012 [Member] \| Interest Rate Received 1.875% [Member]
Derivative [Line Items]
Contractual Notional Amount	$ 0	$ 625
Derivative, Fixed Interest Rate	1.875%	1.875%
Investment Options, Expiration Date	Jan. 15, 2018	Jan. 15, 2018

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

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- Definition

Expiration date of the option.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Footnote 5

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- Definition

Fixed interest rate related to the interest rate derivative.

+ References

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- Definition

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+ References

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