Amounts Recognized for Pension and Postretirement Benefits in Other Comprehensive Income (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Change in total actuarial loss, Before-tax amount	$ (124)	$ 113
Amortization of prior service cost included in net periodic pension and postretirement cost, Before-tax amount	(5)	(5)
Amortization of actuarial loss included in net periodic pension and postretirement cost, Before-tax amount	18	23
Other comprehensive income - pension benefits, Before-tax amount	(111)	131
Change in total actuarial loss, Tax Benefit	35	(41)
Amortization of prior service cost included in net periodic pension and postretirement cost, Tax Benefit	1	2
Amortization of actuarial loss included in net periodic pension and postretirement cost, Tax Benefit	(5)	(7)
Other comprehensive - pension benefits, Tax Benefit	31	(46)
Change in total actuarial loss, Net-of-tax amount	(89)	72
Amortization of prior service cost included in net periodic pension and postretirement cost, Net-of-tax Amount	(4)	(3)
Amortization of actuarial loss included in net periodic pension and postretirement cost, Net-of-tax Amount	13	16
Other comprehensive income - pension benefits, Net-of-tax Amount	$ (80)	$ 85	$ (32)