Schedule of Target Assets Allocation (Parenthetical) (Detail)	12 Months Ended
	Dec. 31, 2023
Bloomberg U.S. Corporate A or Better Index, 20+ Year Index | U.S. corporate fixed income
Defined Benefit Plan Disclosure [Line Items]
Policy index period description	20+ Year
Bloomberg U.S. Corporate A or Better Index, 5- 20 Year Index | U.S. corporate fixed income | Minimum
Defined Benefit Plan Disclosure [Line Items]
Policy index duration period	5 years
Bloomberg U.S. Corporate A or Better Index, 5- 20 Year Index | U.S. corporate fixed income | Maximum
Defined Benefit Plan Disclosure [Line Items]
Policy index duration period	20 years
Bloomberg U.S. 3 - 10 Year Treasury Bond Index | U.S. government fixed income | Minimum
Defined Benefit Plan Disclosure [Line Items]
Policy index duration period	3 years
Bloomberg U.S. 3 - 10 Year Treasury Bond Index | U.S. government fixed income | Maximum
Defined Benefit Plan Disclosure [Line Items]
Policy index duration period	10 years
ICE BofA 3-Month Treasury Bill Index | Cash
Defined Benefit Plan Disclosure [Line Items]
Policy index duration period	3 months