Summary of Accounting Policies - Error Correction, Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net income	$ 111	$ 265	$ 415
Net cash provided by operating activities	439	517	374
Net cash used by investing activities	(2,514)	(300)	(229)
Proceeds from term loans and notes	3,426	160	529
Repayments of term loans and notes	(453)	(36)	(545)
Borrowings on revolving lines of credit	5,149	6,664	5,417
Payments on revolving lines of credit	(5,405)	(6,737)	(5,221)
Net cash provided (used) by financing activities	$ 2,476	(251)	(86)
As Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net income		274	424
Net cash provided by operating activities		629	484
Net cash used by investing activities		(413)	(340)
Retirement of long-term debt		(19)	(531)
Issuance of long-term debt		137	509
Net increase (decrease) in revolver borrowings		(67)	202
Net cash provided (used) by financing activities		(251)	(86)
Revisions
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net income		(9)	(9)
Proceeds from term loans and notes		23	20
Repayments of term loans and notes		(17)	(14)
Borrowings on revolving lines of credit		6,664	5,417
Payments on revolving lines of credit		(6,737)	(5,221)
Net increase (decrease) in revolver borrowings		67	(202)
Reclasses
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Proceeds from term loans and notes		137	509
Repayments of term loans and notes		(19)	(531)
Retirement of long-term debt		19	531
Issuance of long-term debt		(137)	(509)
As Reclassified
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net income		274	424
Proceeds from term loans and notes		137	509
Repayments of term loans and notes		(19)	(531)
Net increase (decrease) in revolver borrowings		(67)	202
Net cash provided (used) by financing activities		$ (251)	$ (86)