Fair Value Measurements (Fair Value And Maximum Future Funding Commitments Related To Investments) (Detail) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Alternative Investment	$ 4,860	$ 4,244
Maximum future funding commitments	3,575	3,705
Financial
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Alternative Investment	575	596
Maximum future funding commitments	$ 375	$ 193
Financial | Minimum [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Liquidating Investment, Remaining Period	2 years	2 years
Financial | Maximum [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Liquidating Investment, Remaining Period	10 years	10 years
Real Assets
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Alternative Investment	$ 776	$ 704
Maximum future funding commitments	$ 542	$ 362
Real Assets | Minimum [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Liquidating Investment, Remaining Period	2 years	2 years
Real Assets | Maximum [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Liquidating Investment, Remaining Period	11 years	11 years
Distressed
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Alternative Investment	$ 267	$ 296
Maximum future funding commitments	$ 93	$ 105
Distressed | Minimum [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Liquidating Investment, Remaining Period	2 years	2 years
Distressed | Maximum [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Liquidating Investment, Remaining Period	7 years	7 years
Private Credit
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Alternative Investment	$ 116	$ 147
Maximum future funding commitments	$ 269	$ 310
Private Credit | Minimum [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Liquidating Investment, Remaining Period	3 years	3 years
Private Credit | Maximum [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Liquidating Investment, Remaining Period	8 years	8 years
Traditional
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Alternative Investment	$ 2,741	$ 2,362
Maximum future funding commitments	$ 2,257	$ 2,735
Traditional | Minimum [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Liquidating Investment, Remaining Period	2 years	2 years
Traditional | Maximum [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Liquidating Investment, Remaining Period	14 years	14 years
Vintage
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Alternative Investment	$ 122	$ 56
Maximum future funding commitments	$ 39	$ 0
Vintage | Minimum [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Liquidating Investment, Remaining Period	1 year	1 year
Vintage | Maximum [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Liquidating Investment, Remaining Period	2 years	2 years
Investment funds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Alternative Investment	$ 263	$ 83
Maximum future funding commitments	$ 0	$ 0