Allowance for Doubtful Accounts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Allowance for Doubtful Accounts Receivable [Roll Forward]
Balance, beginning of period	$ 22	$ 19	$ 14
Charged to expense	135	122	101
Uncollected balances written off, net of recoveries	(136)	(119)	(96)
Balance, end of period	$ 21	$ 22	$ 19