Derivative Instruments - Notional Amounts of Outstanding Derivative Instruments (Details)
¥ in Thousands, $ in Thousands, $ in Thousands |
Jun. 30, 2018
USD ($)
MW
|
Jun. 30, 2018
CAD ($)
MW
|
Jun. 30, 2018
JPY (¥)
MW
|
Dec. 31, 2017
USD ($)
MW
|
Dec. 31, 2017
CAD ($)
MW
|
Dec. 31, 2017
JPY (¥)
MW
|
Dec. 31, 2010
MW
|
| Interest rate swaps [Member] | Designated as Hedging Instrument [Member] |
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| Notional Amount of Outstanding Derivative Contracts [Line Items] |
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| Notional amount |
$ 426,931
|
$ 727,370
|
¥ 55,952,677
|
$ 253,271
|
$ 736,136
|
¥ 0
|
|
| Interest rate swaps [Member] | Not Designated as Hedging Instrument [Member] |
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| Notional Amount of Outstanding Derivative Contracts [Line Items] |
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| Notional amount | $ |
$ 61,806
|
|
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$ 85,474
|
|
|
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| Energy derivative [Member] | Not Designated as Hedging Instrument [Member] |
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| Notional Amount of Outstanding Derivative Contracts [Line Items] |
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| Nonmonetary notional amount | MW |
406,829
|
406,829
|
406,829
|
697,471
|
697,471
|
697,471
|
504,220
|
| Foreign currency forward contracts [Member] | Not Designated as Hedging Instrument [Member] |
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| Notional Amount of Outstanding Derivative Contracts [Line Items] |
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| Notional amount |
|
$ 113,750
|
¥ 12,049,550
|
|
$ 127,500
|
¥ 0
|
|