Investments - Summary of Investments (Details) - USD ($)
$ in Millions |
Mar. 31, 2018 |
Dec. 31, 2017 |
| Schedule of Available-for-sale Securities [Line Items] |
|
|
| Amortized Cost |
$ 2,536
|
$ 2,700
|
| Gross Unrealized Gains |
0
|
1
|
| Gross Unrealized Losses |
15
|
8
|
| Estimated Fair Value |
2,521
|
2,693
|
| Corporate debt securities |
|
|
| Schedule of Available-for-sale Securities [Line Items] |
|
|
| Amortized Cost |
66
|
101
|
| Gross Unrealized Gains |
0
|
0
|
| Gross Unrealized Losses |
0
|
0
|
| Estimated Fair Value |
66
|
101
|
| U.S. treasury notes |
|
|
| Schedule of Available-for-sale Securities [Line Items] |
|
|
| Amortized Cost |
11
|
68
|
| Gross Unrealized Gains |
0
|
0
|
| Gross Unrealized Losses |
0
|
0
|
| Estimated Fair Value |
11
|
68
|
| Subtotal - current restricted investments |
|
|
| Schedule of Available-for-sale Securities [Line Items] |
|
|
| Amortized Cost |
77
|
169
|
| Gross Unrealized Gains |
0
|
0
|
| Gross Unrealized Losses |
0
|
0
|
| Estimated Fair Value |
77
|
169
|
| Corporate debt securities |
|
|
| Schedule of Available-for-sale Securities [Line Items] |
|
|
| Amortized Cost |
1,627
|
1,591
|
| Gross Unrealized Gains |
0
|
1
|
| Gross Unrealized Losses |
9
|
4
|
| Estimated Fair Value |
1,618
|
1,588
|
| U.S. treasury notes |
|
|
| Schedule of Available-for-sale Securities [Line Items] |
|
|
| Amortized Cost |
330
|
389
|
| Gross Unrealized Gains |
0
|
0
|
| Gross Unrealized Losses |
1
|
1
|
| Estimated Fair Value |
329
|
388
|
| GSEs |
|
|
| Schedule of Available-for-sale Securities [Line Items] |
|
|
| Amortized Cost |
229
|
255
|
| Gross Unrealized Gains |
0
|
0
|
| Gross Unrealized Losses |
2
|
2
|
| Estimated Fair Value |
227
|
253
|
| Municipal securities |
|
|
| Schedule of Available-for-sale Securities [Line Items] |
|
|
| Amortized Cost |
133
|
142
|
| Gross Unrealized Gains |
0
|
0
|
| Gross Unrealized Losses |
2
|
1
|
| Estimated Fair Value |
131
|
141
|
| Asset-backed securities |
|
|
| Schedule of Available-for-sale Securities [Line Items] |
|
|
| Amortized Cost |
111
|
117
|
| Gross Unrealized Gains |
0
|
0
|
| Gross Unrealized Losses |
1
|
0
|
| Estimated Fair Value |
110
|
117
|
| Certificate of deposit |
|
|
| Schedule of Available-for-sale Securities [Line Items] |
|
|
| Amortized Cost |
27
|
37
|
| Gross Unrealized Gains |
0
|
0
|
| Gross Unrealized Losses |
0
|
0
|
| Estimated Fair Value |
27
|
37
|
| Other |
|
|
| Schedule of Available-for-sale Securities [Line Items] |
|
|
| Amortized Cost |
2
|
|
| Gross Unrealized Gains |
0
|
|
| Gross Unrealized Losses |
0
|
|
| Estimated Fair Value |
2
|
|
| Subtotal - current investments |
|
|
| Schedule of Available-for-sale Securities [Line Items] |
|
|
| Amortized Cost |
2,459
|
2,531
|
| Gross Unrealized Gains |
0
|
1
|
| Gross Unrealized Losses |
15
|
8
|
| Estimated Fair Value |
$ 2,444
|
$ 2,524
|