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Debt and Capital Leases - Alta Wind Lease Financings Arrangements (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument
Long-term debt	$ 16,633	$ 16,704
Total | Low
Debt Instrument
Interest rate, stated percentage	5.696%
Total | Maximum
Debt Instrument
Interest rate, stated percentage	7.015%
Alta Wind Holdings | Alta Wind I | Leasing Arrangement
Debt Instrument
Long-term debt	$ 231
Interest rate, stated percentage	7.015%
Alta Wind Holdings | Alta Wind I | Letter of Credit
Debt Instrument
Long-term debt	$ 16
Alta Wind Holdings | Alta Wind I | Letter of Credit | Low
Debt Instrument
Interest rate, stated percentage	3.00%
Alta Wind Holdings | Alta Wind I | Letter of Credit | Maximum
Debt Instrument
Interest rate, stated percentage	3.25%
Alta Wind Holdings | Alta Wind II | Leasing Arrangement
Debt Instrument
Long-term debt	$ 183
Interest rate, stated percentage	5.696%
Alta Wind Holdings | Alta Wind II | Letter of Credit
Debt Instrument
Long-term debt	$ 27
Interest rate, stated percentage	1.25%
Alta Wind Holdings | Alta Wind III | Leasing Arrangement
Debt Instrument
Long-term debt	$ 191
Interest rate, stated percentage	6.067%
Alta Wind Holdings | Alta Wind III | Letter of Credit
Debt Instrument
Long-term debt	$ 27
Interest rate, stated percentage	1.75%
Alta Wind Holdings | Alta Wind IV | Leasing Arrangement
Debt Instrument
Long-term debt	$ 123
Interest rate, stated percentage	5.938%
Alta Wind Holdings | Alta Wind IV | Letter of Credit
Debt Instrument
Long-term debt	$ 19
Interest rate, stated percentage	1.75%
Alta Wind Holdings | Alta Wind V | Leasing Arrangement
Debt Instrument
Long-term debt	$ 198
Interest rate, stated percentage	6.071%
Alta Wind Holdings | Alta Wind V | Letter of Credit
Debt Instrument
Long-term debt	$ 30
Interest rate, stated percentage	1.75%
Alta Wind Holdings | Total | Leasing Arrangement
Debt Instrument
Long-term debt	$ 926
Alta Wind Holdings | Total | Letter of Credit
Debt Instrument
Long-term debt	$ 119

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- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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