Fair Value Measurements (Fair Value And Maximum Future Funding Commitments Related To Investments) (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Maximum future funding commitments	$ 3,846	$ 4,061
Fair Value	3,866	3,623
Financial [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Maximum future funding commitments	327	330
Fair Value	$ 538	$ 540
Financial [Member] | 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	5 years	5 years
Financial [Member] | 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	9 years	9 years
Real Assets [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Maximum future funding commitments	$ 210	$ 114
Fair Value	$ 656	$ 651
Real Assets [Member] | 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
Real Assets [Member] | 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
Distressed [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Maximum future funding commitments	$ 131	$ 141
Fair Value	$ 295	$ 289
Distressed [Member] | 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
Distressed [Member] | 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 [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Maximum future funding commitments	$ 320	$ 327
Fair Value	$ 173	$ 187
Private Credit [Member] | 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 [Member] | 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
Traditional [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Maximum future funding commitments	$ 2,858	$ 3,149
Fair Value	$ 1,901	$ 1,656
Traditional [Member] | 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
Traditional [Member] | 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	15 years	15 years
Vintage [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 27	$ 30
Vintage [Member] | 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 [Member] | 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 [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 276	$ 270