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Commitments and Contingencies (Details) - USD ($)
Mar. 31, 2017
Aug. 28, 2008
Performance Guarantee [Member]    
Loss Contingencies [Line Items]    
Liability for guarantees $ 0  
Margin Requirements [Member]    
Loss Contingencies [Line Items]    
Funds drawn under LOC's 0  
Bond Market Fund Litigation [Member]    
Loss Contingencies [Line Items]    
Alleged minimum percentage of fund assets invested in CMOs and mortgage-backed securities without obtaining shareholder vote   25.00%
Standby letters of credit [Member]    
Loss Contingencies [Line Items]    
Aggregate face amount of letter of credit agreements $ 295,000,000