Schedule of Investments (Schedule) (USD $)	Jun. 30, 2012	Dec. 31, 2011
Short-Term Investments U.S. Government and Agency Obligations
Short-Term Investments, Principal Amount	$ 149,860,000
Short-Term Investments, Value	149,832,352
Repurchase Agreements
Repurchase Agreements, Value	13,600,973
Short-Term Investments	163,433,325	175,626,953
8/07/12 [Member] | Federal Home Loan Mortgage Corporation [Member]
Short-Term Investments U.S. Government and Agency Obligations
Short-Term Investments, Principal Amount	30,000,000
Short-Term Investments, Coupon	0.00%
Short-Term Investments, Maturity	Aug. 07, 2012
Short-Term Investments, Ratings	Aaa
Short-Term Investments, Value	29,998,500
8/28/12 [Member] | Federal Home Loan Mortgage Corporation [Member]
Short-Term Investments U.S. Government and Agency Obligations
Short-Term Investments, Principal Amount	18,860,000
Short-Term Investments, Coupon	1.00%
Short-Term Investments, Maturity	Aug. 28, 2012
Short-Term Investments, Ratings	Aaa
Short-Term Investments, Value	18,884,914
9/04/12 [Member] | Federal National Mortgage Association [Member]
Short-Term Investments U.S. Government and Agency Obligations
Short-Term Investments, Principal Amount	21,000,000
Short-Term Investments, Coupon	0.00%
Short-Term Investments, Maturity	Sep. 04, 2012
Short-Term Investments, Ratings	Aaa
Short-Term Investments, Value	20,997,018
11/30/12 [Member] | Federal National Mortgage Association [Member]
Short-Term Investments U.S. Government and Agency Obligations
Short-Term Investments, Principal Amount	20,000,000
Short-Term Investments, Coupon	0.00%
Short-Term Investments, Maturity	Nov. 30, 2012
Short-Term Investments, Ratings	Aaa
Short-Term Investments, Value	19,990,780
8/23/12 [Member] | U.S. Treasury Bills [Member]
Short-Term Investments U.S. Government and Agency Obligations
Short-Term Investments, Principal Amount	15,000,000
Short-Term Investments, Coupon	0.00%
Short-Term Investments, Maturity	Aug. 23, 2012
Short-Term Investments, Ratings	Aaa
Short-Term Investments, Value	14,999,025
11/15/12 [Member] | U.S. Treasury Bills [Member]
Short-Term Investments U.S. Government and Agency Obligations
Short-Term Investments, Principal Amount	25,000,000
Short-Term Investments, Coupon	0.00%
Short-Term Investments, Maturity	Nov. 15, 2012
Short-Term Investments, Ratings	Aaa
Short-Term Investments, Value	24,989,375
2/07/13 [Member] | U.S. Treasury Bills [Member]
Short-Term Investments U.S. Government and Agency Obligations
Short-Term Investments, Principal Amount	10,000,000
Short-Term Investments, Coupon	0.00%
Short-Term Investments, Maturity	Feb. 07, 2013
Short-Term Investments, Ratings	Aaa
Short-Term Investments, Value	9,990,230
5/30/13 [Member] | U.S. Treasury Bills [Member]
Short-Term Investments U.S. Government and Agency Obligations
Short-Term Investments, Principal Amount	10,000,000
Short-Term Investments, Coupon	0.00%
Short-Term Investments, Maturity	May 30, 2013
Short-Term Investments, Ratings	Aaa
Short-Term Investments, Value	9,982,510
7/02/12 [Member]
Repurchase Agreements
Repurchase Agreements, Principal Amount	13,601,000
Repurchase Agreements, Coupon	0.01%
Repurchase Agreements, Maturity	Jul. 02, 2012
Repurchase Agreements, Ratings	N/A
Repurchase Agreements, Value	$ 13,600,973