Acquisition Additional Information (Details)			3 Months Ended		6 Months Ended
	Aug. 10, 2017 USD ($) MW	Apr. 21, 2017 USD ($) kV mi MW	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Jul. 27, 2017 USD ($)	Dec. 31, 2016 USD ($)
Business Acquisition [Line Items]
Equity Method Investments			$ 303,833,000				$ 233,294,000
Long-term Debt, Gross			2,225,857,000				$ 1,605,462,000
Equity Method Investment, Transaction Costs Capitalized			1,500,000
Cost in excess of equity			0
Class B membership interest in Broadview Holdings [Member]
Business Acquisition [Line Items]
Ownership Percentage		100.00%
Western Interconnect [Member]
Business Acquisition [Line Items]
Ownership Percentage		99.00%
Broadview Holdings' Interest in Broadview [Member]
Business Acquisition [Line Items]
Ownership Percentage		100.00%
Initial distributable cash flow from Broadview [Member]
Business Acquisition [Line Items]
Ownership Percentage		84.00%
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners		16.00%
Class A membership interest in Broadview Holdings by NCI [Member]
Business Acquisition [Line Items]
Ownership Percentage		100.00%
Interest in Western Interconnect by third party [Member]
Business Acquisition [Line Items]
Ownership Percentage		1.00%
Broadview Project [Member]
Business Acquisition [Line Items]
Business Acquisition, Percentage of Voting Interests Acquired		100.00%
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Contingent Liability		$ 36,205,000
Payments to Acquire Businesses, Gross		214,700,000
Business Combination, Consideration Transferred, Liabilities Incurred		2,400,000
Business Combination, Contingent Consideration, Liability		21,300,000
Current portion of long-term debt including accrued interest		51,200,000
Business Acquisition, Transaction Costs				$ 400,000	$ 400,000
Decrease in Property, Plant, and Equipment			900,000
Decrease in Accrued Construction Costs			1,200,000
Increase in Asset Retirement Obligations			$ 300,000
Broadview Project [Member] | Continued Operation Of Broadview [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Contingent Liability		7,200,000
Broadview Project [Member] | Continued Operation Of Grady [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Contingent Liability		$ 29,000,000
Meikle [Member]
Business Acquisition [Line Items]
Business Acquisition, Percentage of Voting Interests Acquired	50.99%
Generating Capacity | MW	179
Payments to Acquire Businesses, Gross	$ 67,400,000
Current portion of long-term debt including accrued interest	$ 265,600,000
Meikle Wind Energy Limited Partnership [Member]
Business Acquisition [Line Items]
Business Acquisition, Percentage of Voting Interests Acquired	70.00%
Meikle and Meikle Wind Energy Corp. [Member]
Business Acquisition [Line Items]
Business Combination, Consideration Transferred	$ 67,400,000.0
Business Acquisition, Transaction Costs Capitalized	$ 1,100,000
Pattern Development 2.0 [Member]
Business Acquisition [Line Items]
Equity Method Investments						$ 60,000,000
Equity Method Investment, Ownership Percentage						20.00%
Western Interconnect [Member]
Business Acquisition [Line Items]
Transmission line length | mi		35
Transmission-level voltage | kV		345
Long-term Debt, Gross		$ 54,400,000.0
Broadview [Member]
Business Acquisition [Line Items]
Generating Capacity | MW		324
Public Sector Pension Investment Board [Member] | Meikle [Member]
Business Acquisition [Line Items]
Business Acquisition, Percentage of Voting Interests Acquired	48.99%
Public Sector Pension Investment Board [Member] | Meikle Wind Energy Limited Partnership [Member]
Business Acquisition [Line Items]
Business Acquisition, Percentage of Voting Interests Acquired	30.00%
Public Sector Pension Investment Board [Member] | Meikle and Meikle Wind Energy Corp. [Member]
Business Acquisition [Line Items]
Business Combination, Consideration Transferred	$ 64,800,000.0