Retirement Plans (Projected Benefit Obligation) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Change in plan assets [Roll Forward]
Fair value of plan assets at beginning of year	$ 1,572
Fair value of plan assets acquired	1,108
Fair value of plan assets at end of year	2,766	$ 1,572
Amounts recognized in the consolidated balance sheet [Abstract]
Pension and other postretirement benefits - current	(23)	(33)
Pension and other postretirement benefits - noncurrent	(1,602)	(1,163)
Pension Benefits [Member]
Change in projected benefit obligation (PBO) [Roll Forward]
PBO at beginning of year	2,142	2,210
Service cost	88	55	$ 42
Interest cost	122	88	80
Actuarial (gain)/loss	137	(88)
Benefits paid	(155)	(128)
Connecticut Acquisition Transfer		5
CTF Acquisition PBO	1,108
Defined Benefit Plan, Special Termination Benefits	23
PBO at end of year	3,465	2,142	2,210
Change in plan assets [Roll Forward]
Fair value of plan assets at beginning of year	1,572	1,673
Actual return on plan assets	201	(40)
Employer contributions	28	62	83
Benefits paid	(155)	(128)
Fair value of plan assets at end of year	2,766	1,572	1,673
Funded status	(699)	(570)
Amounts recognized in the consolidated balance sheet [Abstract]
Pension and other postretirement benefits - current		(9)
Pension and other postretirement benefits - noncurrent	(699)	(561)
Accumulated other comprehensive (gain) loss	647	584
Pension Benefits [Member] | Verizon Transaction
Change in plan assets [Roll Forward]
Fair value of plan assets acquired	1,120
Pension Benefits [Member] | Connecticut Transaction
Change in plan assets [Roll Forward]
Fair value of plan assets acquired		5
Postretirement Benefits Other Than Pensions (OPEB) [Member]
Change in projected benefit obligation (PBO) [Roll Forward]
PBO at beginning of year	626	727
Service cost	19	19	11
Interest cost	37	30	22
Actuarial (gain)/loss	(18)	(115)
Benefits paid	(23)	(25)
Connecticut Acquisition Transfer		5
CTF Acquisition PBO	276
Defined Benefit Plan, Special Termination Benefits	3
Plan participants' contributions	5	5
Plan change		(20)
PBO at end of year	925	626	$ 727
Change in plan assets [Roll Forward]
Plan participant's contributions	5	5
Employer contributions	18	20
Benefits paid	(23)	(25)
Funded status	(925)	(626)
Amounts recognized in the consolidated balance sheet [Abstract]
Pension and other postretirement benefits - current	(23)	(24)
Pension and other postretirement benefits - noncurrent	(902)	(602)
Accumulated other comprehensive (gain) loss	$ (29)	$ (20)