Shareholders' Equity (Summary Of Changes In Accumulated Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated other comprehensive income [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax	$ (139)				$ (122)				$ (139)	$ (122)	$ (55)
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Tax									19	(45)
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Tax									(1)	0
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, Tax									11	5
Other Comprehensive Income (Loss), Tax									29	(40)
Other comprehensive income (loss)									(17)	(67)	73
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax									6	(65)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax									2	(2)
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount									(47)	0	0
Effect of Change in Tax Rate, OCI									25	0	0
Income Tax Expense (Benefit)									344	236	142
Net Income (Loss) Available to Common Stockholders, Basic	114	$ 153	$ 154	$ 129	125	$ 117	$ 90	$ 79	550	411	247
Reclassification of amortization of net actuarial gains losses pension and post retirement from AOCI, gross									(7)	(8)	(6)
Reclassification of pension and postretirement from accumulated other comprehensive income, tax									(3)	(3)	(2)
Accumulated Net Unrealized Investment Gain (Loss) [Member]
Accumulated other comprehensive income [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax	(114)				(92)				(114)	(92)	(18)
Other comprehensive income (loss)									(22)	(74)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax									(2)	(74)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax									0	0	86
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount									(20)
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]
Accumulated other comprehensive income [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax	(3)				1				(3)	1	1
Other comprehensive income (loss)									(4)	0
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax									(1)	7
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax									(2)	(7)
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount									(1)
Net Income (Loss) Available to Common Stockholders, Basic									2	7	6
Accumulated Defined Benefit Plans Adjustment [Member]
Accumulated other comprehensive income [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax	$ (22)				$ (31)				(22)	(31)	(38)
Other comprehensive income (loss)									9	7
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax									9	2
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax									4	5	4
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount									(4)
Fixed income securities gain loss [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Unrealized Investment Gain (Loss) [Member]
Accumulated other comprehensive income [Line Items]
Gain (Loss) on Investments, Excluding Other than Temporary Impairments									0	0	(139)
Income Tax Expense (Benefit)									0	0	(53)
Interest Rate Contract [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]
Accumulated other comprehensive income [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax									4	11	9
Income Tax Expense (Benefit)									$ 2	$ 4	$ 3