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Retirement Benefits - Fair Value Measurements (Details) - USD ($)
$ in Millions
Mar. 31, 2019
Mar. 31, 2018
Mar. 31, 2017
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets $ 480.2 $ 507.4  
Alternative Investment 430.1 461.7  
Level 1      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 20.2 15.5  
Level 2      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Level 3      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 29.9 30.2  
Cash and cash equivalents      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 20.2 5.9  
Alternative Investment 0.0 3.7  
Cash and cash equivalents | Level 1      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 20.2 2.2  
Cash and cash equivalents | Level 2      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Cash and cash equivalents | Level 3      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets   0.0  
Fixed income funds      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 304.6 328.0  
Alternative Investment 304.6 317.3  
Fixed income funds | Level 1      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 10.7  
Fixed income funds | Level 2      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Fixed income funds | Level 3      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
US equity funds      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 54.3 61.9  
Alternative Investment 54.3 59.6  
US equity funds | Level 1      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 2.3  
US equity funds | Level 2      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
US equity funds | Level 3      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
International equity funds      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 33.4 35.9  
Alternative Investment 33.4 35.6  
International equity funds | Level 1      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.3  
International equity funds | Level 2      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
International equity funds | Level 3      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Balanced funds      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 6.4 9.5  
Alternative Investment 6.4 9.5  
Balanced funds | Level 1      
Defined Benefit Plan Disclosure [Line Items]      
Alternative Investment 0.0 0.0  
Balanced funds | Level 2      
Defined Benefit Plan Disclosure [Line Items]      
Alternative Investment 0.0 0.0  
Balanced funds | Level 3      
Defined Benefit Plan Disclosure [Line Items]      
Alternative Investment 0.0 0.0  
Alternative Investments      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 31.4 36.0  
Alternative Investment 31.4 36.0  
Alternative Investments | Level 1      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Alternative Investments | Level 2      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Alternative Investments | Level 3      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Insurance contracts      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 29.9 30.2  
Alternative Investment 0.0 0.0  
Insurance contracts | Level 1      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Insurance contracts | Level 2      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Insurance contracts | Level 3      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets $ 29.9 $ 30.2 $ 23.9