Shareholders' Equity - Components of Accumulated Other Comprehensive Income (Loss) (Parenthetical) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Net Unrealized Gains (Losses) on Investments with OTTI Losses [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Tax on change in other comprehensive income (loss) before reclassifications	$ 3	$ 2	$ 1
Tax on reclassification from accumulated other comprehensive income	(3)	2	1
Net Other Unrealized Gains (Losses) on Investments [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Tax on change in other comprehensive income (loss) before reclassifications	(256)	213	(106)
Tax on reclassification from accumulated other comprehensive income	1	(2)	38
Unrealized Gains (Losses) on Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Tax on change in other comprehensive income (loss) before reclassifications	5	(2)	(2)
Tax on reclassification from accumulated other comprehensive income	0	0	0
Pension and Postretirement Benefits [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Tax on change in other comprehensive income (loss) before reclassifications	28	9	4
Tax on reclassification from accumulated other comprehensive income	(9)	(6)	(16)
Foreign Currency Translation [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Tax on change in other comprehensive income (loss) before reclassifications	0	0	0
Tax on reclassification from accumulated other comprehensive income	$ 0	0	$ 0
Accounting Standards Update 2016-01 [Member] | Net Unrealized Gains (Losses) on Investments with OTTI Losses [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Tax on cumulative effect adjustment for adoption of ASU 2016-01		0
Accounting Standards Update 2016-01 [Member] | Net Other Unrealized Gains (Losses) on Investments [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Tax on cumulative effect adjustment for adoption of ASU 2016-01		8
Accounting Standards Update 2016-01 [Member] | Unrealized Gains (Losses) on Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Tax on cumulative effect adjustment for adoption of ASU 2016-01		0
Accounting Standards Update 2016-01 [Member] | Pension and Postretirement Benefits [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Tax on cumulative effect adjustment for adoption of ASU 2016-01		0
Accounting Standards Update 2016-01 [Member] | Foreign Currency Translation [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Tax on cumulative effect adjustment for adoption of ASU 2016-01		$ 0