Commitments, Contingencies and Guarantees - External Credit Ratings of Underlying or Referenced Assets for Credit Related Derivatives Contracts (Detail) $ in Millions	May 31, 2017 USD ($)
Index credit default swaps [Member]
Credit Derivatives [Line Items]
External credit ratings	$ 864.3
Single name credit default swaps [Member]
Credit Derivatives [Line Items]
External credit ratings	209.9
Below Investment Grade [Member] | Index credit default swaps [Member]
Credit Derivatives [Line Items]
External credit ratings	107.3
Below Investment Grade [Member] | Single name credit default swaps [Member]
Credit Derivatives [Line Items]
External credit ratings	150.3
AAA/Aaa [Member] | Index credit default swaps [Member]
Credit Derivatives [Line Items]
External credit ratings	0.0
AAA/Aaa [Member] | Single name credit default swaps [Member]
Credit Derivatives [Line Items]
External credit ratings	0.0
AA/Aa [Member] | Index credit default swaps [Member]
Credit Derivatives [Line Items]
External credit ratings	0.0
AA/Aa [Member] | Single name credit default swaps [Member]
Credit Derivatives [Line Items]
External credit ratings	0.0
A [Member] | Index credit default swaps [Member]
Credit Derivatives [Line Items]
External credit ratings	757.0
A [Member] | Single name credit default swaps [Member]
Credit Derivatives [Line Items]
External credit ratings	15.3
BBB/Baa [Member] | Index credit default swaps [Member]
Credit Derivatives [Line Items]
External credit ratings	0.0
BBB/Baa [Member] | Single name credit default swaps [Member]
Credit Derivatives [Line Items]
External credit ratings	$ 44.3