Retirement Plans - Fair Value Based on Level 3 Inputs Held by Institutional Funds (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plan Disclosure [Line Items]
Ending balance	$ 2,251	$ 2,145
Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Beginning balance	430	377
Actual return on plan assets	24	39
Purchases, sales or other settlements	3	14
Ending balance	457	430
Fixed Income [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Beginning balance	3	2
Purchases, sales or other settlements	(1)	1
Ending balance	2	3
Hedge Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Ending balance	302	291
Hedge Funds [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Beginning balance	291	255
Actual return on plan assets	9	26
Purchases, sales or other settlements	2	10
Ending balance	302	291
Private Equity Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Ending balance	105	89
Private Equity Funds [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Beginning balance	89	75
Actual return on plan assets	15	11
Purchases, sales or other settlements	1	3
Ending balance	105	89
Real Estate Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Ending balance	60	57
Real Estate Funds [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Beginning balance	47	45
Actual return on plan assets		2
Purchases, sales or other settlements	1
Ending balance	$ 48	$ 47