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Other long-term liabilities - Schedule of Long-Term Liabilities and Deferred Credits (Detail) - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Other Long-term Liabilities |  |  |
Advances in aid of construction (a) | $ 63,703 | $ 62,683 |
Environmental remediation obligation (b) | 55,621 | 54,322 | $ 47,202
Asset retirement obligations (c) | 43,291 | 44,166 | $ 18,486
Customer deposits (d) | 29,974 | 28,529 |
Unamortized investment tax credits (e) | 17,491 | 17,839 |
Deferred credits (f) | 42,711 | 21,168 |
Preferred shares, Series C (g) | 13,418 | 14,718 |
Other (h) | 39,710 | 45,434 |
Other Liabilities | 305,919 | 288,859 |
Less: current portion | (42,337) | (46,754) |
Other long-term liabilities | 263,582 | 242,105 |
Transfers from advances in aid of construction to contributions in aid of construction | 3,687 | 10,498 |
Estimated environmental remediation costs | 59,181 | 57,292 |
Accrual for environmental loss contingencies to be incurred over next three years | $ 36,611 |  |
Accrual for environmental loss contingencies, payment period | 27 years |  |
Amount of regulatory assets | $ 450,474 | 441,526 |
Minimum |  |  |
Other Long-term Liabilities |  |  |
Other liability repayment period | 5 years |  |
Accrual environmental cost | 2.50% |  |
Maximum |  |  |
Other Long-term Liabilities |  |  |
Other liability repayment period | 40 years |  |
Accrual environmental cost | 2.80% |  |
Environmental costs |  |  |
Other Long-term Liabilities |  |  |
Regulatory asset, expenditure recovery provided by regulator, time period | 7 years |  |
Amount of regulatory assets | $ 82,295 | $ 82,711 |
Series C Preferred Stock |  |  |
Other Long-term Liabilities |  |  |
Preferred shares, Series C (g) | $ 13,418 |  |

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