Schedule of Investments (Bond Index 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
Bond Index Fund
Investment And Investment Type [Line Items]
Value	99,988,000
Liabilities Less Other Assets	(94,080)
Net Assets	99,893,920	100,316,764	88,287,000
Bond Index Fund \| Investment Funds
Investment And Investment Type [Line Items]
Value	99,988,000
Investments at cost	92,570,592
Percentage of investment held in investment contracts	100.10%
Bond Index Fund \| Investment Funds \| Collective Investment
Investment And Investment Type [Line Items]
Percentage of investment held in investment contracts	100.10%
Bond Index Fund \| Investment Funds \| Collective Investment \| SSgA U.S. Bond Index Non-Lending Series Fund, Class A
Investment And Investment Type [Line Items]
Shares	7,976,069
Value	99,988,000
Bond Index Fund \| Investments
Investment And Investment Type [Line Items]
Value	99,988,000
Investments at cost	$ 92,570,592
Percentage of investment held in investment contracts	100.10%
Bond Index Fund \| Liabilities Less Other Assets
Investment And Investment Type [Line Items]
Percentage of investment held in investment contracts	(0.10%)
Bond Index 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.

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+ Details

- Definition

Balance held at close of period in number of shares.

+ References+ Details

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