Debt - Revolving Credit and Letter of Credit (Details) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2018 USD ($) Banks	Jun. 30, 2018 USD ($) Banks	Mar. 31, 2018 USD ($)
Bridge Loan [Member]
Line of Credit Facility [Line Items]
Number of participating banks | Banks	23	23
Credit agreement term		364 days
Maximum borrowing capacity	$ 23,700	$ 23,700	$ 26,700
Leverage ratio covenant	60.00%	60.00%
Fees incurred			$ 140
Fees paid		$ 111
Amortization of fees	$ 65	$ 85
Revolving Credit And Letter Of Credit Facility [Member]
Line of Credit Facility [Line Items]
Number of participating banks | Banks	23	23
Credit agreement term		5 years
Maximum borrowing capacity	$ 1,500	$ 1,500
Maximum borrowing capacity on or after the Merger	3,250	3,250
Amount by which credit facilty amount can be increased	$ 500	$ 500
Amount by which credit facilty term length can be increased		1 year
Leverage ratio covenant	50.00%	50.00%
Leverage ratio covenant post-Merger	60.00%	60.00%
Letter of Credit [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 500	$ 500
Maximum borrowing capacity on or after the Merger	$ 500	$ 500
Term Loan Credit Agreement [Member]
Line of Credit Facility [Line Items]
Number of participating banks | Banks	26	26
Credit agreement term		3 years
Aggregate principal amount	$ 3,000	$ 3,000
Leverage ratio covenant	60.00%	60.00%