Postretirement Benefit Plans - Additional Information (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Liability related to funded defined-benefit pension plans	$ 408,000,000	$ 406,000,000
Cash contributions made to benefit plans	0
Insurance cash value and Rabbi Trust investments supporting supplemental pension plan liability	81,000,000	79,000,000
Non-qualified supplemental retirement pension plan liability	68,000,000	$ 67,000,000
Maximum [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected cash contributions	$ 20,000,000