Debt - Revolving Credit and Letter of Credit (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) Banks	Apr. 06, 2018 USD ($) Banks
Bridge Loan [Member]
Line of Credit Facility [Line Items]
Number of participating banks | Banks	23
Credit agreement term	364 days
Maximum borrowing capacity	$ 26,700
Leverage ratio covenant	60.00%
Fees incurred	$ 140
Fees paid	70
Amortization of fees	$ 20
Bridge Loan [Member] | Subsequent Event [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity		$ 23,700
Revolving Credit And Letter Of Credit Facility [Member]
Line of Credit Facility [Line Items]
Credit agreement term	5 years
Amount by which credit facilty term length can be increased	1 year
Revolving Credit And Letter Of Credit Facility [Member] | Subsequent Event [Member]
Line of Credit Facility [Line Items]
Number of participating banks | Banks		23
Maximum borrowing capacity		$ 1,500
Maximum borrowing capacity on or after the Merger		3,250
Amount by which credit facilty amount can be increased		$ 500
Leverage ratio covenant		50.00%
Leverage ratio covenant post-Merger		60.00%
Letter of Credit [Member] | Subsequent Event [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity		$ 500
Maximum borrowing capacity on or after the Merger		$ 500
Term Loan Credit Agreement [Member]
Line of Credit Facility [Line Items]
Credit agreement term	3 years
Term Loan Credit Agreement [Member] | Subsequent Event [Member]
Line of Credit Facility [Line Items]
Number of participating banks | Banks		26
Aggregate principal amount		$ 3,000
Leverage ratio covenant		60.00%