Financial instruments - Liabilities Mature (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Derivative [Line Items]
Long-term debt obligations	$ 4,534,031
Interest on long-term debt	1,884,209
Purchase obligations	561,690
Environmental obligation	66,133
Advances in aid of construction	79,864	$ 60,828
Other obligations	216,261
Total obligations	7,452,598
Cross currency swap
Derivative [Line Items]
Derivative financial instruments, liabilities	84,542
Interest rate swaps
Derivative [Line Items]
Derivative financial instruments, liabilities	19,325
Energy derivative and commodity contracts
Derivative [Line Items]
Derivative financial instruments, liabilities	6,543
Due less than 1 year
Derivative [Line Items]
Long-term debt obligations	334,352
Interest on long-term debt	195,876
Purchase obligations	561,690
Environmental obligation	16,955
Advances in aid of construction	1,236
Other obligations	79,219
Total obligations	1,231,308
Due less than 1 year \| Cross currency swap
Derivative [Line Items]
Derivative financial instruments, liabilities	37,338
Due less than 1 year \| Interest rate swaps
Derivative [Line Items]
Derivative financial instruments, liabilities	2,725
Due less than 1 year \| Energy derivative and commodity contracts
Derivative [Line Items]
Derivative financial instruments, liabilities	1,917
Due 2 to 3 years
Derivative [Line Items]
Long-term debt obligations	821,535
Interest on long-term debt	337,199
Purchase obligations	0
Environmental obligation	26,409
Advances in aid of construction	0
Other obligations	6,601
Total obligations	1,225,856
Due 2 to 3 years \| Cross currency swap
Derivative [Line Items]
Derivative financial instruments, liabilities	29,999
Due 2 to 3 years \| Interest rate swaps
Derivative [Line Items]
Derivative financial instruments, liabilities	4,346
Due 2 to 3 years \| Energy derivative and commodity contracts
Derivative [Line Items]
Derivative financial instruments, liabilities	(233)
Due 4 to 5 years
Derivative [Line Items]
Long-term debt obligations	285,600
Interest on long-term debt	267,112
Purchase obligations	0
Environmental obligation	1,251
Advances in aid of construction	0
Other obligations	5,232
Total obligations	584,358
Due 4 to 5 years \| Cross currency swap
Derivative [Line Items]
Derivative financial instruments, liabilities	19,875
Due 4 to 5 years \| Interest rate swaps
Derivative [Line Items]
Derivative financial instruments, liabilities	4,369
Due 4 to 5 years \| Energy derivative and commodity contracts
Derivative [Line Items]
Derivative financial instruments, liabilities	919
Due after 5 years
Derivative [Line Items]
Long-term debt obligations	3,092,544
Interest on long-term debt	1,084,022
Purchase obligations	0
Environmental obligation	21,518
Advances in aid of construction	78,628
Other obligations	125,209
Total obligations	4,411,076
Due after 5 years \| Cross currency swap
Derivative [Line Items]
Derivative financial instruments, liabilities	(2,670)
Due after 5 years \| Interest rate swaps
Derivative [Line Items]
Derivative financial instruments, liabilities	7,885
Due after 5 years \| Energy derivative and commodity contracts
Derivative [Line Items]
Derivative financial instruments, liabilities	$ 3,940

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Other Noncurrent Obligations

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Undiscounted amount of the accrual for environmental loss contingencies.

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Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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Amount of long-term debt and lease obligation, classified as noncurrent.

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