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Commitments and contingencies - Estimates of Future Commitments (Detail) - USD ($) $ in Thousands |  |  |  | 12 Months Ended
--- | --- | --- | --- | ---
Oct. 30, 2018 | Jun. 22, 2017 | May 06, 2014 | Dec. 31, 2018
Commitments Disclosure [Line Items] |  |  |  |
Year 1 |  |  |  | $ 243,130
Year 2 |  |  |  | 110,155
Year 3 |  |  |  | 84,529
Year 4 |  |  |  | 87,629
Year 5 |  |  |  | 81,503
Thereafter |  |  |  | 730,663
Total |  |  |  | 1,337,609
Power purchase |  |  |  |
Commitments Disclosure [Line Items] |  |  |  |
Year 1 |  |  |  | 46,536
Year 2 |  |  |  | 10,896
Year 3 |  |  |  | 11,114
Year 4 |  |  |  | 11,338
Year 5 |  |  |  | 11,566
Thereafter |  |  |  | 191,208
Total |  |  |  | 282,658
Gas supply and service agreements |  |  |  |
Commitments Disclosure [Line Items] |  |  |  |
Year 1 |  |  |  | 77,658
Year 2 |  |  |  | 51,349
Year 3 |  |  |  | 27,672
Year 4 |  |  |  | 24,422
Year 5 |  |  |  | 22,424
Thereafter |  |  |  | 48,313
Total |  |  |  | 251,838
Service agreements |  |  |  |
Commitments Disclosure [Line Items] |  |  |  |
Year 1 |  |  |  | 43,732
Year 2 |  |  |  | 39,093
Year 3 |  |  |  | 38,451
Year 4 |  |  |  | 37,463
Year 5 |  |  |  | 40,737
Thereafter |  |  |  | 312,559
Total |  |  |  | 512,035
Capital projects |  |  |  |
Commitments Disclosure [Line Items] |  |  |  |
Year 1 |  |  |  | 67,575
Year 2 |  |  |  | 1,663
Year 3 |  |  |  | 196
Year 4 |  |  |  | 7,330
Year 5 |  |  |  | 0
Thereafter |  |  |  | 0
Total |  |  |  | 76,764
Operating leases |  |  |  |
Commitments Disclosure [Line Items] |  |  |  |
Year 1 |  |  |  | 7,629
Year 2 |  |  |  | 7,154
Year 3 |  |  |  | 7,096
Year 4 |  |  |  | 7,076
Year 5 |  |  |  | 6,776
Thereafter |  |  |  | 178,583
Total |  |  |  | 214,314
Gaia Power Inc. vs APUC |  |  |  |
Commitments Disclosure [Line Items] |  |  |  |
Damages claimed by other party in lawsuit | $ 345,000 |  |  |
Punitive damages claimed by other party in lawsuit | $ 25,000 |  |  |
Lawsuit vs. Mountain Water Company and City of Missoula |  |  |  |
Commitments Disclosure [Line Items] |  |  |  |
Fair value of condemned property |  |  | $ 88,600 |
Litigation settlement, amount |  | $ 83,863 |  |
Gain on long-lived assets |  |  |  | $ 4,370

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