Fair Value Disclosures - Investments in Certain Entities Calculate Net Asset Value per Share (Parenthetical) (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Total Unfunded Commitments	$ 309	$ 286
Hedge Funds/Funds of Hedge Funds [Member] \| Equity Method Investments [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Liquidation period, years	7 years	7 years
Total Unfunded Commitments	$ 64	$ 48
Private Equity Funds [Member] \| Equity Method Investments [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Liquidation period, years	6 years	6 years
Total Unfunded Commitments	$ 85	$ 86
Real Assets Funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Total Unfunded Commitments	128	117
Amount of unfunded commitments contractually obligated to fund	108	98
Real Assets Funds [Member] \| Consolidated Variable Interest Entities [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Total Unfunded Commitments	$ 48	$ 49
Real Assets Funds [Member] \| Equity Method Investments [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Redemption period	8 years	8 years
Total Unfunded Commitments	$ 79	$ 69
Private Equity Funds Of Funds [Member] \| Consolidated Variable Interest Entities [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Liquidation period, years	5 years		5 years
Total Unfunded Commitments	$ 18	20
Amount of unfunded commitments contractually obligated to fund	$ 23	$ 23