Revenue from contracts with customers (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Contract with Customer, Asset and Liability [Abstract]
Receivables, revenue from contracts with customers	$ 44,704		$ 44,704		$ 30,563
Contract assets - current	4,270		4,270		3,787
Contract assets - non-current	23,525		23,525		19,474
Total contract assets	27,795		27,795		23,261
Contract liability	7,889		7,889		6,542
Revenue recognized in the year from [Abstract]
Amounts included in contract liability at the beginning of the year					4,816	$ 0
Unbilled receivables	6,999		6,999
Unbilled receivables included in other current assets	356		356
Unbilled receivables included in other non-current asset	6,643		6,643		0
Disaggregation of Revenue [Abstract]
Operating revenue	76,177	$ 31,738	139,679	$ 57,876
Other revenues	18,389	8,028	29,417	11,841
Revenue, Performance Obligation Satisfied over Time [Abstract]
Remaining performance obligation	4,410,159		4,410,159
Capitalized Contract Cost [Abstract]
Capitalized contract costs	10,031		10,031		8,839
Other current assets	498		498		331
Other noncurrent assets	9,533		9,533		$ 8,508
Power and Steam [Member]
Disaggregation of Revenue [Abstract]
Operating revenue	6,946		8,677
Development Services [Member]
Disaggregation of Revenue [Abstract]
Other revenues	17,495	3,908	27,566	3,908
Lease and Other Revenue [Member]
Disaggregation of Revenue [Abstract]
Other revenues	894	$ 4,120	1,851	$ 7,933
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-07-01
Revenue, Performance Obligation Satisfied over Time [Abstract]
Remaining performance obligation	$ 129,090		$ 129,090
Remaining performance obligation, expected timing of satisfaction, period	6 months		6 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01
Revenue, Performance Obligation Satisfied over Time [Abstract]
Remaining performance obligation	$ 243,553		$ 243,553
Remaining performance obligation, expected timing of satisfaction, period	1 year		1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01
Revenue, Performance Obligation Satisfied over Time [Abstract]
Remaining performance obligation	$ 242,745		$ 242,745
Remaining performance obligation, expected timing of satisfaction, period	1 year		1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-01-01
Revenue, Performance Obligation Satisfied over Time [Abstract]
Remaining performance obligation	$ 242,836		$ 242,836
Remaining performance obligation, expected timing of satisfaction, period	1 year		1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2024-01-01
Revenue, Performance Obligation Satisfied over Time [Abstract]
Remaining performance obligation	$ 243,090		$ 243,090
Remaining performance obligation, expected timing of satisfaction, period	1 year		1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2025-01-01
Revenue, Performance Obligation Satisfied over Time [Abstract]
Remaining performance obligation	$ 3,308,845		$ 3,308,845
Remaining performance obligation, expected timing of satisfaction, period