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Stock-based Compensation (PitchBook Bonus Plan) (Details) - Performance share awards [Member]
$ / shares in Units, $ in Millions
12 Months Ended 36 Months Ended
Dec. 31, 2019
USD ($)
$ / shares
shares
Dec. 31, 2019
USD ($)
$ / shares
shares
PitchBook Plan, For 2017 To 2019 [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Shares available for issuance, aggregate target value $ 30.0 $ 30.0
Target value of shares expected to be granted in year one 7.5 7.5
Target value of shares expected to be granted in year two 7.5 7.5
Target value of shares expected to be granted in year three 15.0 15.0
PitchBook Plan, Renewal For 2020 To 2022 [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Shares available for issuance, aggregate target value 30.0 30.0
Target value of shares expected to be granted in year one 7.5 7.5
Target value of shares expected to be granted in year two 7.5 7.5
Target value of shares expected to be granted in year three $ 15.0 $ 15.0
PitchBook Plan [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Award vesting period 1 year  
Shares of common stock to be issued for each equity instrument | shares 1 1
Target performance share awards granted | shares   132,522
Weighted average fair value per award | $ / shares $ 112.16 $ 112.16
Number of shares that will be issued based on final 2019 performance levels | shares   182,211
Unamortized expense, based on current performance levels (in millions) $ 0.0 $ 0.0