Commitments, Contingencies and Guarantees (External Credit Ratings of Underlying or Referenced Assets for Credit Related Derivatives Contracts) (Details) $ in Millions	Jun. 30, 2017 USD ($)
Commitments And Guarantee Obligations [Line Items]
Maximum Payout	$ 8,447.2
Below Investment Grade [Member] | Index credit default swaps [Member]
Commitments And Guarantee Obligations [Line Items]
Credit related derivative contracts by external credit ratings	107.3
Below Investment Grade [Member] | Single name credit default swaps [Member]
Commitments And Guarantee Obligations [Line Items]
Credit related derivative contracts by external credit ratings	150.3
AAA/Aaa [Member] | Index credit default swaps [Member]
Commitments And Guarantee Obligations [Line Items]
Credit related derivative contracts by external credit ratings	0.0
AAA/Aaa [Member] | Single name credit default swaps [Member]
Commitments And Guarantee Obligations [Line Items]
Credit related derivative contracts by external credit ratings	0.0
AA/Aa [Member] | Index credit default swaps [Member]
Commitments And Guarantee Obligations [Line Items]
Credit related derivative contracts by external credit ratings	0.0
AA/Aa [Member] | Single name credit default swaps [Member]
Commitments And Guarantee Obligations [Line Items]
Credit related derivative contracts by external credit ratings	0.0
A [Member] | Index credit default swaps [Member]
Commitments And Guarantee Obligations [Line Items]
Credit related derivative contracts by external credit ratings	757.0
A [Member] | Single name credit default swaps [Member]
Commitments And Guarantee Obligations [Line Items]
Credit related derivative contracts by external credit ratings	15.3
BBB/Baa [Member] | Index credit default swaps [Member]
Commitments And Guarantee Obligations [Line Items]
Credit related derivative contracts by external credit ratings	0.0
BBB/Baa [Member] | Single name credit default swaps [Member]
Commitments And Guarantee Obligations [Line Items]
Credit related derivative contracts by external credit ratings	44.3
Notional/Maximum Payout [Member] | Index credit default swaps [Member]
Commitments And Guarantee Obligations [Line Items]
Maximum Payout	864.3
Notional/Maximum Payout [Member] | Single name credit default swaps [Member]
Commitments And Guarantee Obligations [Line Items]
Maximum Payout	$ 209.9