Assets and Liabilities Measured at Fair Value on a Recurring Basis (Additional Information) (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Equity Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Investments, Fair Value Disclosure	$ 49	$ 102
Investment in Federal Home Loan Bank Stock [Member] | Fair Value, Inputs, Level 3 [Member]
Investments, Fair Value Disclosure	15	132
Federal Reserve Bank Stock [Member] | Fair Value, Inputs, Level 3 [Member]
Investments, Fair Value Disclosure	403	402
Equity Securities [Member]
Investments, Fair Value Disclosure	$ 2	$ 6