Financial Instruments And Fair Value Measurements - Fair Value Measurements on a Recurring Basis (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Total	$ 1,778	$ 1,287
Certificate of deposit
Schedule of Available-for-sale Securities [Line Items]
Total	2	2
Fair Value Measurements on a Recurring Basis
Schedule of Available-for-sale Securities [Line Items]
Restricted cash equivalents	220	140
Restricted investments	40	54
Total	260	194
Fair Value Measurements on a Recurring Basis | U.S. treasuries
Schedule of Available-for-sale Securities [Line Items]
Restricted investments	37	51
Fair Value Measurements on a Recurring Basis | Exchange traded fund
Schedule of Available-for-sale Securities [Line Items]
Restricted investments	1	1
Fair Value Measurements on a Recurring Basis | Certificate of deposit
Schedule of Available-for-sale Securities [Line Items]
Restricted investments	2	2
Fair Value Measurements on a Recurring Basis | Money market mutual funds
Schedule of Available-for-sale Securities [Line Items]
Restricted cash equivalents	199	117
Fair Value Measurements on a Recurring Basis | Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Restricted cash equivalents	21	23
Fair Value Measurements on a Recurring Basis | Level 1
Schedule of Available-for-sale Securities [Line Items]
Restricted cash equivalents	220	140
Restricted investments	38	52
Total	258	192
Fair Value Measurements on a Recurring Basis | Level 1 | U.S. treasuries
Schedule of Available-for-sale Securities [Line Items]
Restricted investments	37	51
Fair Value Measurements on a Recurring Basis | Level 1 | Exchange traded fund
Schedule of Available-for-sale Securities [Line Items]
Restricted investments	1	1
Fair Value Measurements on a Recurring Basis | Level 1 | Certificate of deposit
Schedule of Available-for-sale Securities [Line Items]
Restricted investments	0	0
Fair Value Measurements on a Recurring Basis | Level 1 | Money market mutual funds
Schedule of Available-for-sale Securities [Line Items]
Restricted cash equivalents	199	117
Fair Value Measurements on a Recurring Basis | Level 1 | Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Restricted cash equivalents	21	23
Fair Value Measurements on a Recurring Basis | Level 2
Schedule of Available-for-sale Securities [Line Items]
Restricted cash equivalents	0	0
Restricted investments	2	2
Total	2	2
Fair Value Measurements on a Recurring Basis | Level 2 | U.S. treasuries
Schedule of Available-for-sale Securities [Line Items]
Restricted investments	0	0
Fair Value Measurements on a Recurring Basis | Level 2 | Exchange traded fund
Schedule of Available-for-sale Securities [Line Items]
Restricted investments	0	0
Fair Value Measurements on a Recurring Basis | Level 2 | Certificate of deposit
Schedule of Available-for-sale Securities [Line Items]
Restricted investments	2	2
Fair Value Measurements on a Recurring Basis | Level 2 | Money market mutual funds
Schedule of Available-for-sale Securities [Line Items]
Restricted cash equivalents	0	0
Fair Value Measurements on a Recurring Basis | Level 2 | Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Restricted cash equivalents	$ 0	$ 0