Fair Value Measurements (Fair Value And Maximum Future Funding Commitments Related To Investments) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 3,623	$ 3,627
Maximum future funding commitment	4,061	2,040
Financial
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	540	548
Maximum future funding commitment	$ 330	$ 428
Financial | Minimum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Expected liquidation period	5 years	5 years
Financial | 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]
Fair Value	$ 651	$ 536
Maximum future funding commitment	$ 114	$ 230
Real Assets | Minimum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Expected liquidation period	3 years	3 years
Real Assets | Maximum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Expected liquidation period	7 years	7 years
Distressed
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 289	$ 485
Maximum future funding commitment	$ 141	$ 179
Distressed | Minimum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Expected liquidation period	3 years	3 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]
Fair Value	$ 187	$ 236
Maximum future funding commitment	$ 327	$ 259
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	7 years	7 years
Traditional
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 1,656	$ 1,550
Maximum future funding commitment	$ 3,149	$ 930
Traditional | Minimum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Expected liquidation period	3 years	3 years
Traditional | Maximum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Expected liquidation period	15 years	15 years
Vintage
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 30	$ 21
Maximum future funding commitment		$ 14
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]
Fair Value	$ 270	$ 251