Accumulated Other Comprehensive Income (Loss) by Component (Schedule of Accumulated Other Comprehensive Income (Loss) by Component) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2022
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Total stockholder's equity at beginning of period	$ 9,893
Other comprehensive income (loss) before reclassifications	69	$ 265
Amounts reclassified from accumulated other comprehensive income (loss) net of tax	(31)	(25)
Other comprehensive income (loss) after tax (expense) benefit	100	290
Total stockholder's equity at end of period	9,662	8,667
Cumulative effect adjustments from changes in accounting guidance, tax			$ 11
Reclassification from AOCI, tax	8	6
Tax (expense) benefit on other comprehensive income (loss)	(35)	(75)
Accumulated other comprehensive income (loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Total stockholder's equity at beginning of period	(2,672)	(3,598)
Total stockholder's equity at end of period	(2,572)	(3,308)	(3,598)
Accumulated other comprehensive income (loss) | Previously Reported
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Total stockholder's equity at beginning of period	(2,672)	(3,557)
Total stockholder's equity at end of period			(3,557)
Accumulated other comprehensive income (loss) | Revision of Prior Period, Accounting Standards Update, Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Total stockholder's equity at beginning of period	0	(41)
Total stockholder's equity at end of period			(41)
Net unrealized gains (losses) on investments | Net unrealized gains (losses) on investments with an allowance for credit losses
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Total stockholder's equity at beginning of period	(12)	(7)
Other comprehensive income (loss) before reclassifications	(1)	(8)
Amounts reclassified from accumulated other comprehensive income (loss) net of tax	(3)	0
Other comprehensive income (loss) after tax (expense) benefit	2	(8)
Total stockholder's equity at end of period	(10)	(15)	(7)
Cumulative effect adjustments from changes in accounting guidance, tax			0
Reclassification from AOCI, tax	1	0
Tax (expense) benefit on other comprehensive income (loss)	(1)	2
Net unrealized gains (losses) on investments | Net unrealized gains (losses) on investments with an allowance for credit losses | Previously Reported
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Total stockholder's equity at beginning of period		(7)
Total stockholder's equity at end of period			(7)
Net unrealized gains (losses) on investments | Net unrealized gains (losses) on investments with an allowance for credit losses | Revision of Prior Period, Accounting Standards Update, Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Total stockholder's equity at beginning of period		0
Total stockholder's equity at end of period			0
Net unrealized gains (losses) on investments | Net unrealized gains and losses on other investments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Total stockholder's equity at beginning of period	(1,613)	(2,738)
Other comprehensive income (loss) before reclassifications	(239)	652
Amounts reclassified from accumulated other comprehensive income (loss) net of tax	(22)	(18)
Other comprehensive income (loss) after tax (expense) benefit	(217)	670
Total stockholder's equity at end of period	(1,830)	(2,068)	(2,738)
Cumulative effect adjustments from changes in accounting guidance, tax			0
Reclassification from AOCI, tax	6	4
Tax (expense) benefit on other comprehensive income (loss)	58	(180)
Net unrealized gains (losses) on investments | Net unrealized gains and losses on other investments | Previously Reported
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Total stockholder's equity at beginning of period		(2,738)
Total stockholder's equity at end of period			(2,738)
Net unrealized gains (losses) on investments | Net unrealized gains and losses on other investments | Revision of Prior Period, Accounting Standards Update, Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Total stockholder's equity at beginning of period		0
Total stockholder's equity at end of period			0
Pension and postretirement benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Total stockholder's equity at beginning of period	(525)	(591)
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive income (loss) net of tax	(6)	(7)
Other comprehensive income (loss) after tax (expense) benefit	6	7
Total stockholder's equity at end of period	(519)	(584)	(591)
Cumulative effect adjustments from changes in accounting guidance, tax			0
Reclassification from AOCI, tax	1	2
Tax (expense) benefit on other comprehensive income (loss)	(1)	(2)
Pension and postretirement benefits | Previously Reported
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Total stockholder's equity at beginning of period		(591)
Total stockholder's equity at end of period			(591)
Pension and postretirement benefits | Revision of Prior Period, Accounting Standards Update, Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Total stockholder's equity at beginning of period		0
Total stockholder's equity at end of period			0
Cumulative impact of changes in discount rates used to measure long duration contracts
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Total stockholder's equity at beginning of period	(359)	(41)
Other comprehensive income (loss) before reclassifications	341	(396)
Amounts reclassified from accumulated other comprehensive income (loss) net of tax	0	0
Other comprehensive income (loss) after tax (expense) benefit	341	(396)
Total stockholder's equity at end of period	(18)	(437)	(41)
Cumulative effect adjustments from changes in accounting guidance, tax			11
Reclassification from AOCI, tax	0	0
Tax (expense) benefit on other comprehensive income (loss)	(91)	105
Cumulative impact of changes in discount rates used to measure long duration contracts | Previously Reported
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Total stockholder's equity at beginning of period		0
Total stockholder's equity at end of period			0
Cumulative impact of changes in discount rates used to measure long duration contracts | Revision of Prior Period, Accounting Standards Update, Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Total stockholder's equity at beginning of period		(41)
Total stockholder's equity at end of period			(41)
Cumulative foreign currency translation adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Total stockholder's equity at beginning of period	(163)	(221)
Other comprehensive income (loss) before reclassifications	(32)	17
Amounts reclassified from accumulated other comprehensive income (loss) net of tax	0	0
Other comprehensive income (loss) after tax (expense) benefit	(32)	17
Total stockholder's equity at end of period	(195)	(204)	(221)
Cumulative effect adjustments from changes in accounting guidance, tax			0
Reclassification from AOCI, tax	0	0
Tax (expense) benefit on other comprehensive income (loss)	$ 0	0
Cumulative foreign currency translation adjustment | Previously Reported
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Total stockholder's equity at beginning of period		(221)
Total stockholder's equity at end of period			(221)
Cumulative foreign currency translation adjustment | Revision of Prior Period, Accounting Standards Update, Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Total stockholder's equity at beginning of period		$ 0
Total stockholder's equity at end of period			$ 0