Acquisitions (Purchase Price Allocation) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Business Acquisition [Line Items]
Goodwill	$ 9,674	$ 7,166	$ 7,205
Verizon Transaction
Business Acquisition [Line Items]
Current assets	350
Property, plant and equipment	6,236
Goodwill	2,508
Other intangibles - customer base	2,162
Current liabilities	(518)
Long-term debt	(544)
Other liabilities	(323)
Total net assets acquired	$ 9,871
Connecticut Transaction
Business Acquisition [Line Items]
Current assets		69
Property, plant and equipment		1,459
Goodwill		815
Other intangibles - customer base		570
Current liabilities		(94)
Deferred income taxes		(576)
Other liabilities		(225)
Total net assets acquired		$ 2,018