v3.19.3.a.u2
Summary of significant accounting policies - through derivatives (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2019
USD ($)
item
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Accounts receivable      
Allowance for doubtful accounts $ 0.0 $ 0.0  
Deferred financing costs      
Net carrying amount of deferred financing cost 8.5 11.8  
Amortization expense $ 3.2 $ 5.1 $ 6.3
Impairment of long-lived assets, non-amortizing intangible assets and equity method investments      
Period prior to expiration of PPA when impairment review is conducted 6 months    
Derivative financial instruments      
Number of interest rate swaps designated as cash flow hedges | item 1    
Minimum      
Deferred financing costs      
Amortization period 1 year    
Investments accounted for by the equity method      
Ownership level triggering equity method of accounting, as a percent 5.00%    
Maximum      
Deferred financing costs      
Amortization period 6 years    
Investments accounted for by the equity method      
Ownership level triggering equity method of accounting, as a percent 50.00%    
Impairment of long-lived assets, non-amortizing intangible assets and equity method investments      
Maximum percentage of ownership in entities in which consolidation is not required 50.00%