v3.10.0.1
STOCKHOLDERS' EQUITY - compensation expense (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Recorded stock-based compensation expense $ 4.2 $ 10.9
Unrecognized stock-based compensation expense   18.5
2018 Board of Directors    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Recorded stock-based compensation expense   0.5
2018 RSU awards    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Recorded stock-based compensation expense 1.0 2.4
Unrecognized stock-based compensation expense   7.8
2018 PSU awards    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Recorded stock-based compensation expense 1.8 4.1
Unrecognized stock-based compensation expense   6.0
2017 RSU awards    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Recorded stock-based compensation expense 0.5 1.3
Unrecognized stock-based compensation expense   2.4
2017 RSU NEO awards    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Recorded stock-based compensation expense 0.3 1.0
Unrecognized stock-based compensation expense   1.7
2016 RSU awards    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Recorded stock-based compensation expense 0.3 0.8
Unrecognized stock-based compensation expense   0.3
2016 RSU NEO awards    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Recorded stock-based compensation expense $ 0.3 0.8
Unrecognized stock-based compensation expense   0.3
Expected to Recognize 2018    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock-based compensation expense expected to be recognized   4.2
Expected to Recognize 2018 | 2018 RSU awards    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock-based compensation expense expected to be recognized   1.0
Expected to Recognize 2018 | 2018 PSU awards    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock-based compensation expense expected to be recognized   1.8
Expected to Recognize 2018 | 2017 RSU awards    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock-based compensation expense expected to be recognized   0.5
Expected to Recognize 2018 | 2017 RSU NEO awards    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock-based compensation expense expected to be recognized   0.3
Expected to Recognize 2018 | 2016 RSU awards    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock-based compensation expense expected to be recognized   0.3
Expected to Recognize 2018 | 2016 RSU NEO awards    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock-based compensation expense expected to be recognized   0.3
Expected to Recognize 2019    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock-based compensation expense expected to be recognized   9.7
Expected to Recognize 2019 | 2018 RSU awards    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock-based compensation expense expected to be recognized   3.4
Expected to Recognize 2019 | 2018 PSU awards    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock-based compensation expense expected to be recognized   3.0
Expected to Recognize 2019 | 2017 RSU awards    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock-based compensation expense expected to be recognized   1.9
Expected to Recognize 2019 | 2017 RSU NEO awards    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock-based compensation expense expected to be recognized   1.4
Expected to Recognize 2020    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock-based compensation expense expected to be recognized   4.6
Expected to Recognize 2020 | 2018 RSU awards    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock-based compensation expense expected to be recognized   3.4
Expected to Recognize 2020 | 2018 PSU awards    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock-based compensation expense expected to be recognized   $ 1.2