Table:
|  | Years Ended December 31, |  |  |
| --- | --- | --- | --- |
|  | 2019 | 2018 | 2017 |
|  | (Dollars in millions) |  |  |
| Balance at beginning of year | $190 | 115 | 95 |
| Accretion expense | 11 | 10 | 6 |
| Liabilities assumed in acquisition of Level 3(1) | — | 58 | 45 |
| Liabilities settled | (14) | (14) | (3) |
| Liabilities transferred to Cyxtera | — | — | (20) |
| Change in estimate | 10 | 21 | (8) |
| Balance at end of year | $197 | 190 | 115 |
Paragraphs:
Asset Retirement Obligations
At December 31, 2019, our asset retirement obligations balance was primarily related to estimated future costs of removing equipment from leased properties and estimated future costs of properly disposing of asbestos and other hazardous materials upon remodeling or demolishing buildings. Asset retirement obligations are included in other long-term liabilities on our consolidated balance sheets.
As of the Level 3 acquisition date, we recorded liabilities to reflect our fair values of Level 3’s asset retirement obligations. Our fair value estimates were determined using the discounted cash flow method.
The following table provides asset retirement obligation activity:
(1) The liabilities assumed during 2018 relate to purchase price adjustments during the year.
The 2019, 2018 and 2017 change in estimates are offset against gross property, plant and equipment.
Question:
What is the average accretion expense across 2017, 2018 and 2019?
Answer: (11+10+6)/3 = 9