Pension Plans, Retirement Benefits and Savings Plans (details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pension Plans [Member]
Defined benefit plan disclosure
Service cost	$ 30	$ 30	$ 60	$ 59
Interest cost on benefit obligation	30	31	61	61
Expected return on plan assets	(60)	(58)	(120)	(115)
Net periodic benefit cost, settlement		1		1
Net periodic benefit cost, amortization of unrecognized net actuarial loss	19	16	37	33
Total non-service cost	(11)	(10)	(22)	(20)
Net periodic benefit cost	19	20	38	39
Pension Plans [Member] | Claims and claim adjustment expenses [Member]
Defined benefit plan disclosure
Service cost	12	12	24	24
Total non-service cost	(5)	(5)	(9)	(9)
Pension Plans [Member] | General and administrative expenses [Member]
Defined benefit plan disclosure
Service cost	18	18	36	35
Total non-service cost	(6)	(5)	(13)	(11)
Postretirement Benefit Plans [Member]
Defined benefit plan disclosure
Interest cost on benefit obligation	1	2	3	4
Net periodic benefit cost, amortization of unrecognized prior service benefit	$ (1)	(1)	(2)	(2)
Total non-service cost		1	1	2
Net periodic benefit cost		1	1	2
Postretirement Benefit Plans [Member] | Claims and claim adjustment expenses [Member]
Defined benefit plan disclosure
Total non-service cost		$ 1		1
Postretirement Benefit Plans [Member] | General and administrative expenses [Member]
Defined benefit plan disclosure
Total non-service cost			$ 1	$ 1