Revenue from contracts - Contract balances (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Accounts receivable	$ 30.4	$ 35.7
Contract liabilities	0.3	0.1
Fuel reserve fund	0.2
San Diego Projects
Disaggregation of Revenue [Line Items]
Contract liabilities	$ 0.1	$ 0.1

Amount, after allowance for credit loss, of right to consideration from customer for product sold and service rendered in normal course of business, classified as current.

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant