Commitments and Contingencies (Details) - USD ($)	Mar. 31, 2018	Aug. 28, 2008
Loss Contingencies [Line Items]
Aggregate face amount of letter of credit agreements	$ 225,000,000
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] | CMOs and Mortgage-backed Securities [Member]
Loss Contingencies [Line Items]
Alleged minimum percentage of fund assets invested in CMOs and mortgage-backed securities without obtaining shareholder vote, more than		25.00%