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Employee Share Ownership Plans - Summary of Description of Plans (Parenthetical) (Detail)
 12 Months Ended
Jun. 30, 2018
USD ($)
STIP and GSTIP [member]  
Disclosure of terms and conditions of share-based payment arrangement [Line Items]  
Vesting period 2 years
Shareplus [member]  
Disclosure of terms and conditions of share-based payment arrangement [Line Items]  
Vesting period 3 years
Maximum employee contributions to acquire shares $ 5,000
Long-Term Incentive Plan [member]  
Disclosure of terms and conditions of share-based payment arrangement [Line Items]  
Vesting percentage per year 5.50%
Vesting period 5 years
Long-Term Incentive Plan [member] | Peer Group Total Shareholder Return [Member]  
Disclosure of terms and conditions of share-based payment arrangement [Line Items]  
Vesting percentage 67.00%
Long-Term Incentive Plan [member] | Index Total Shareholder Return [Member]  
Disclosure of terms and conditions of share-based payment arrangement [Line Items]  
Vesting percentage 33.00%
Bottom of range [Member] | Transitional OMC awards [member]  
Disclosure of terms and conditions of share-based payment arrangement [Line Items]  
Vesting period 3 years
Bottom of range [Member] | MAP awards [member]  
Disclosure of terms and conditions of share-based payment arrangement [Line Items]  
Vesting period 1 year
Top of range [Member] | Transitional OMC awards [member]  
Disclosure of terms and conditions of share-based payment arrangement [Line Items]  
Vesting period 4 years
Top of range [Member] | MAP awards [member]  
Disclosure of terms and conditions of share-based payment arrangement [Line Items]  
Vesting period 5 years