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Debt (Narrative - 5.875% Senior Notes) (Details) - USD ($)
$ in Millions
12 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Debt Instrument [Line Items]    
Coupon rate 5.875%  
Principal amount of debt outstanding $ 2,520.0 $ 3,107.5
5.875% Senior Notes    
Debt Instrument [Line Items]    
Discount Rate, Basis Spread on Treasury Rate 0.50%  
Net Income, Amount Free of Restrictions $ 468.1  
Retained Earnings, Amount Free of Restrictions $ 516.6  
5.875% Senior Notes | Senior Notes    
Debt Instrument [Line Items]    
Coupon rate 5.875%  
Principal amount of debt outstanding $ 520.0 $ 520.0
Debt instrument redemption premium percentage, as a percentage of principal amount prepaid or redeemed 1.00%  
Prior to November 1, 2019 | 5.875% Senior Notes | Senior Notes    
Debt Instrument [Line Items]    
Debt instrument redemption price percentage 100.00%  
On or after November 1, 2019 | 5.875% Senior Notes | Senior Notes    
Debt Instrument [Line Items]    
Debt instrument redemption price percentage 104.406%  
On or after November 1, 2020 | 5.875% Senior Notes | Senior Notes    
Debt Instrument [Line Items]    
Debt instrument redemption price percentage 102.938%  
On or after November 1, 2021 | 5.875% Senior Notes | Senior Notes    
Debt Instrument [Line Items]    
Debt instrument redemption price percentage 101.439%  
On or after November 1, 2022 | 5.875% Senior Notes | Senior Notes    
Debt Instrument [Line Items]    
Debt instrument redemption price percentage 100.00%  
Change in Control | 5.875% Senior Notes | Senior Notes    
Debt Instrument [Line Items]    
Debt instrument redemption price percentage 101.00%  
Certain Asset Disposition | 5.875% Senior Notes | Senior Notes    
Debt Instrument [Line Items]    
Debt instrument redemption price percentage 100.00%