Schedule of Investments (Real Asset Return Fund ) (Detail) (USD $)	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012
Investment And Investment Type [Line Items]
Investments at cost	$ 2,132,135,165	$ 2,138,687,993
Net Assets	4,355,480,521	4,137,647,057
Real Asset Return Fund
Investment And Investment Type [Line Items]
Value	31,983,985
Liabilities Less Other Assets	(32,838)
Net Assets	31,951,147	31,516,328	25,498,000
Real Asset Return Fund \| Investment Funds
Investment And Investment Type [Line Items]
Value	31,983,985
Investments at cost	30,132,581
Percentage of investment held in investment contracts	100.10%
Real Asset Return Fund \| Investment Funds \| Collective Investment Funds
Investment And Investment Type [Line Items]
Percentage of investment held in investment contracts	100.10%
Real Asset Return Fund \| Investment Funds \| Collective Investment Funds \| SSgA Dow Jones UBS-Commodity IndexSM Non-Lending Series Fund, Class A
Investment And Investment Type [Line Items]
Shares	948,276
Value	7,938,018
Real Asset Return Fund \| Investment Funds \| Collective Investment Funds \| SSgA U.S. Inflation Protected Bond Index Non-Lending Series Fund, Class A
Investment And Investment Type [Line Items]
Shares	634,122
Value	15,882,226
Real Asset Return Fund \| Investment Funds \| Collective Investment Funds \| SSgA/Tuckerman REIT Index Non-Lending Series Fund, Class A
Investment And Investment Type [Line Items]
Shares	221,768
Value	8,163,741
Real Asset Return Fund \| Investments
Investment And Investment Type [Line Items]
Value	31,983,985
Investments at cost	$ 30,132,581
Percentage of investment held in investment contracts	100.10%
Real Asset Return Fund \| Liabilities Less Other Assets
Investment And Investment Type [Line Items]
Percentage of investment held in investment contracts	(0.10%)
Real Asset Return Fund \| Net Assets
Investment And Investment Type [Line Items]
Percentage of investment held in investment contracts	100.00%

Value of the investment at close of period. For schedules of investments that are categorized, the value would be aggregated by category. For investment in and advances to affiliates, if operations of any controlled companies are different in character from those of the company, group such affiliates within divisions and by type of activities.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

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Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

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Reference 5: http://www.xbrl.org/2003/role/presentationRef

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Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

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Reference 7: http://www.xbrl.org/2003/role/presentationRef

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-Publisher SEC

-Name Regulation S-X (SX)

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Reference 9: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 13

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Reference 10: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

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Reference 11: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 12

-Paragraph 3, 4

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+ Details

- Definition

Balance held at close of period in number of shares.

+ References+ Details

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