Commitments, Contingencies And Guarantees (External Credit Ratings Of Underlying Or Referenced Assets For Credit Related Derivatives Contracts) (Details) $ in Millions	Mar. 31, 2017 USD ($)
Commitments And Guarantee Obligations [Line Items]
Maximum Payout	$ 7,234.4
Below Investment Grade [Member] | Index credit default swaps [Member]
Commitments And Guarantee Obligations [Line Items]
Credit related derivative contracts by external credit ratings	0.0
Below Investment Grade [Member] | Single name credit default swaps [Member]
Commitments And Guarantee Obligations [Line Items]
Credit related derivative contracts by external credit ratings	180.2
AAA/Aaa [Member] | Index credit default swaps [Member]
Commitments And Guarantee Obligations [Line Items]
Credit related derivative contracts by external credit ratings	4.0
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	0.0
A [Member] | Single name credit default swaps [Member]
Commitments And Guarantee Obligations [Line Items]
Credit related derivative contracts by external credit ratings	115.1
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	59.9
Notional/Maximum Payout [Member] | Index credit default swaps [Member]
Commitments And Guarantee Obligations [Line Items]
Maximum Payout	4.0
Notional/Maximum Payout [Member] | Single name credit default swaps [Member]
Commitments And Guarantee Obligations [Line Items]
Maximum Payout	$ 355.2