Fair Value Measurements (Fair Value And Maximum Future Funding Commitments Related To Investments) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Alternative Investment	$ 4,244	$ 3,623
Maximum future funding commitment	3,705	4,061
Financial Service [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Alternative Investment	596	540
Maximum future funding commitment	$ 193	$ 330
Financial Service [Member] | Minimum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Expected liquidation period	2 years	2 years
Financial Service [Member] | Maximum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Expected liquidation period	9 years	9 years
Real Assets
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Alternative Investment	$ 704	$ 651
Maximum future funding commitment	$ 362	$ 114
Real Assets | Minimum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Expected liquidation period	2 years	2 years
Real Assets | Maximum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Expected liquidation period	11 years	11 years
Distressed
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Alternative Investment	$ 296	$ 289
Maximum future funding commitment	$ 105	$ 141
Distressed | Minimum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Expected liquidation period	2 years	2 years
Distressed | Maximum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Expected liquidation period	7 years	7 years
Private Credit
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Alternative Investment	$ 147	$ 187
Maximum future funding commitment	$ 310	$ 327
Private Credit | Minimum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Expected liquidation period	3 years	3 years
Private Credit | Maximum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Expected liquidation period	8 years	8 years
Traditional
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Alternative Investment	$ 2,362	$ 1,656
Maximum future funding commitment	$ 2,735	$ 3,149
Traditional | Minimum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Expected liquidation period	2 years	2 years
Traditional | Maximum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Expected liquidation period	14 years	14 years
Vintage
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Alternative Investment	$ 56	$ 30
Vintage | Minimum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Expected liquidation period	1 year	1 year
Vintage | Maximum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Expected liquidation period	2 years	2 years
Investment Funds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Alternative Investment	$ 83	$ 270