Shareholders' Equity - Components of Accumulated Other Comprehensive Income (Loss) (Parenthetical) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
OTTI Gains (Losses) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Tax on change in other comprehensive income (loss) before reclassifications	$ 2	$ 1	$ (4)
Tax on reclassification from accumulated other comprehensive loss	2	1	3
Unrealized Gains (Losses) on Investments [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Tax on change in other comprehensive income (loss) before reclassifications	213	(106)	(133)
Tax on reclassification from accumulated other comprehensive loss	(2)	38	16
Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Tax on change in other comprehensive income (loss) before reclassifications	(2)	(2)	0
Tax on reclassification from accumulated other comprehensive loss	0	0	0
Pension Liability [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Tax on change in other comprehensive income (loss) before reclassifications	9	4	9
Tax on reclassification from accumulated other comprehensive loss	(6)	(16)	(15)
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 loss	0	$ 0	$ 0
Accounting Standards Update 2016-01 [Member] | OTTI Gains (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] | 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] | 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 Liability [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