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Equity Incentive Plan - Performance unit awards (Details) - Performance unit awards
$ / shares in Units, $ in Millions
3 Months Ended
Mar. 31, 2021
USD ($)
$ / shares
March 5, 2020  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Requisite service period of the awards 3 years
Combined expense per award (USD per share) $ 41.82
March 9, 2021  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Stock-based compensation not yet recognized | $ $ 7.0
Recognition period for costs not yet recognized 2 years 7 months 13 days
Combined expense per award (USD per share) $ 39.77
Minimum | March 5, 2020  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Payout range (as a percent) 0.00%
Minimum | March 9, 2021  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Payout rate of market criteria awards (as a percent) 0.00%
Payout rate of performance criteria awards (as a percent) 0.00%
Maximum | March 5, 2020  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Payout range (as a percent) 200.00%
Payout range if ATSR Appreciation is zero or less (as a percent) 100.00%
Maximum | March 9, 2021  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Payout rate of market criteria awards (as a percent) 250.00%
Payout rate of performance criteria awards (as a percent) 200.00%