Financial Instruments and Fair Value Measurements - Summary of Financial Instruments by Significant Categories and Fair Value Measurement on a Recurring Basis (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Financial Instruments [Line Items]
Total restricted cash equivalents and investments	$ 1,211	$ 1,778
Certificate of deposit
Fair Value, Financial Instruments [Line Items]
Total restricted cash equivalents and investments	2	2
Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Restricted cash equivalents	244	220
Restricted investments	26	40
Total restricted cash equivalents and investments	270	260
Fair Value Measurement on a Recurring Basis | Level 1
Fair Value, Financial Instruments [Line Items]
Restricted cash equivalents	244	220
Restricted investments	24	38
Total restricted cash equivalents and investments	268	258
Fair Value Measurement on a Recurring Basis | Level 2
Fair Value, Financial Instruments [Line Items]
Restricted cash equivalents	0	0
Restricted investments	2	2
Total restricted cash equivalents and investments	2	2
Fair Value Measurement on a Recurring Basis | U.S. treasuries
Fair Value, Financial Instruments [Line Items]
Restricted investments	23	37
Fair Value Measurement on a Recurring Basis | U.S. treasuries | Level 1
Fair Value, Financial Instruments [Line Items]
Restricted investments	23	37
Fair Value Measurement on a Recurring Basis | U.S. treasuries | Level 2
Fair Value, Financial Instruments [Line Items]
Restricted investments	0	0
Fair Value Measurement on a Recurring Basis | Exchange traded fund
Fair Value, Financial Instruments [Line Items]
Restricted investments	1	1
Fair Value Measurement on a Recurring Basis | Exchange traded fund | Level 1
Fair Value, Financial Instruments [Line Items]
Restricted investments	1	1
Fair Value Measurement on a Recurring Basis | Exchange traded fund | Level 2
Fair Value, Financial Instruments [Line Items]
Restricted investments	0	0
Fair Value Measurement on a Recurring Basis | Certificate of deposit
Fair Value, Financial Instruments [Line Items]
Restricted investments	2	2
Fair Value Measurement on a Recurring Basis | Certificate of deposit | Level 1
Fair Value, Financial Instruments [Line Items]
Restricted investments	0	0
Fair Value Measurement on a Recurring Basis | Certificate of deposit | Level 2
Fair Value, Financial Instruments [Line Items]
Restricted investments	2	2
Fair Value Measurement on a Recurring Basis | Money market mutual funds
Fair Value, Financial Instruments [Line Items]
Restricted cash equivalents	224	199
Fair Value Measurement on a Recurring Basis | Money market mutual funds | Level 1
Fair Value, Financial Instruments [Line Items]
Restricted cash equivalents	224	199
Fair Value Measurement on a Recurring Basis | Money market mutual funds | Level 2
Fair Value, Financial Instruments [Line Items]
Restricted cash equivalents	0	0
Fair Value Measurement on a Recurring Basis | Commercial paper
Fair Value, Financial Instruments [Line Items]
Restricted cash equivalents	20	21
Fair Value Measurement on a Recurring Basis | Commercial paper | Level 1
Fair Value, Financial Instruments [Line Items]
Restricted cash equivalents	20	21
Fair Value Measurement on a Recurring Basis | Commercial paper | Level 2
Fair Value, Financial Instruments [Line Items]
Restricted cash equivalents	$ 0	$ 0