Debt - Revolving Credit and Letter of Credit (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) Banks	Dec. 20, 2018 USD ($) Banks	Mar. 31, 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
Fees incurred	$ 140
Revolving Credit And Letter Of Credit Facility [Member]
Line of Credit Facility [Line Items]
Number of participating banks | Banks	23
Expiration date	Apr. 06, 2023
Maximum borrowing capacity	$ 3,250
Amount by which credit facilty amount can be increased	$ 500
Leverage ratio covenant	60.00%
Debt covenant compliance	The Company was in compliance with its debt covenants as of December 31, 2018.
Letter of Credit [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 500
Letters of credit outstanding	$ 22
Term Loan Credit Agreement [Member]
Line of Credit Facility [Line Items]
Number of participating banks | Banks		26
Aggregate principal amount		$ 3,000
Remaining amount available for borrowing		$ 0
Leverage ratio covenant		60.00%