Postretirement Benefit Plans (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Defined Benefit Plan Disclosure [Line Items]
Disclosure of other pension plans includes pension plans with pension assets or obligations in excess of threshold amount	$ 50
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected return on plan assets	(801)	(875)
Service cost for benefits earned	318	401
Interest cost on benefit obligation	686	614
Prior service cost amortization	54	61
Net actuarial (gain) loss amortization	641	912
Pension plans cost	898	1,113
Other Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected return on plan assets	(197)	(165)
Service cost for benefits earned	114	102
Interest cost on benefit obligation	146	130
Prior service cost amortization	1	2
Net actuarial (gain) loss amortization	49	86
Pension plans cost	113	155
Principal Retiree Health and Life Insurance Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected return on plan assets	(12)	(15)
Service cost for benefits earned	44	73
Interest cost on benefit obligation	110	107
Prior service cost amortization	98	98
Net actuarial (gain) loss amortization	(43)	4
Pension plans cost	$ 197	$ 267