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Label Element Value
Risk Return [Abstract] rr_RiskReturnAbstract  
ProspectusDate rr_ProspectusDate Dec. 30, 2011
Preferred Securities Fund (Fourth Prospectus Summary) | Preferred Securities Fund
  
Risk Return [Abstract] rr_RiskReturnAbstract  
Risk/Return, Heading rr_RiskReturnHeading Preferred Securities Fund
Investment Objective, Heading rr_ObjectiveHeading Objective:
investment Objective, Primary rr_ObjectivePrimaryTextBlock 
The Fund seeks to provide current income.
Expense, Heading rr_ExpenseHeading Fees and Expenses of the Fund
Expense, Narrative rr_ExpenseNarrativeTextBlock 
This table describes the fees and expenses that you may pay if you buy and hold
shares of the Fund.
Shareholder Fees, Caption rr_ShareholderFeesCaption Shareholder Fees (fees paid directly from your investment): None
Operating Expenses, Caption rr_OperatingExpensesCaption Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
Portfolio Turnover, Heading rr_PortfolioTurnoverHeading Portfolio Turnover
Portfolio Turnover rr_PortfolioTurnoverTextBlock 
The Fund pays transaction costs, such as commissions, when it buys and sells
securities (or "turns over" its portfolio). A higher portfolio turnover rate may
indicate higher transaction costs and may result in higher taxes when Fund
shares are held in a taxable account. These costs, which are not reflected in
annual fund operating expenses or in the example, affect the Fund's performance.
During the most recent fiscal year, the Fund's annualized portfolio turnover
rate was 16.8% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 16.80%
Expense Example, Heading rr_ExpenseExampleHeading Example
Expense Example, Narrative rr_ExpenseExampleNarrativeTextBlock 
This Example is intended to help you compare the cost of investing in the Fund
with the cost of investing in other mutual funds.

The Example assumes that you invest $10,000 in the Fund for the time periods
indicated and then redeem all of your shares at the end of those periods. The
Example also assumes that your investment has a 5% return each year and that the
Fund's operating expenses remain the same. Although your actual costs may be
higher or lower, based on these assumptions your costs would be:
Investment Strategy, Heading rr_StrategyHeading Principal Investment Strategies
Investment Strategy, Narrative rr_StrategyNarrativeTextBlock 
Under normal circumstances, the Fund invests at least 80% of its net assets in
preferred securities at the time of purchase. Preferred securities generally pay
fixed rate dividends (though some are adjustable rate) and typically have
"preference" over common stock in the payment of dividends and the liquidation
of a company's assets, but are junior to all forms of the company's debt. Most
of the securities purchased by the Fund are preferred securities of companies
rated at the time of purchase BBB- or higher by Standard & Poor's Rating Service
("S&P") or Baa3 or higher by Moody's Investor Service, Inc. ("Moody's") or, if
unrated, of comparable quality in the opinion of the Sub-Advisor. The Fund also
invests up to 15% of its assets in high yield, below investment grade quality
debt (sometimes called "junk bonds" and rated at the time of purchase BB+ or
lower by S&P or rated Ba1 or lower by Moody's or of equivalent quality as
determined by the Sub-Advisor).

The Fund may concentrate its investments (invest more than 25% of its net
assets) in securities in the U.S. and non-U.S financial services (i.e., banking,
insurance and commercial finance,) industry. The Fund also regularly invests in
the real estate investment trust (i.e. REIT) and utility industries.
Investment Strategy, Portfolio Concentration rr_StrategyPortfolioConcentration The Fund may concentrate its investments (invest more than 25% of its net assets) in securities in the U.S. and non-U.S financial services (i.e., banking, insurance and commercial finance,) industry.
Risk, Heading rr_RiskHeading Principal Risks
Risk, Narrative rr_RiskNarrativeTextBlock 
The Fund may be an appropriate investment for investors who are seeking
dividends to generate income or to reinvest for growth and are willing to accept
fluctuations in the value of the investment.

The value of your investment in the Fund changes with the value of the Fund's
investments. Many factors affect that value, and it is possible to lose money by
investing in the Fund. An investment in the Fund is not a deposit of a bank and
is not insured or guaranteed by the Federal Deposit Insurance Corporation or any
other government agency. The principal risks of investing in the Fund, in
alphabetical order, are:

Equity Securities Risk. Equity securities (common, convertible preferred stocks
and other securities whose values are tied to the price of stocks, such as
rights, warrants and convertible debt securities) could decline in value if the
issuer's financial condition declines or in response to overall market and
economic conditions. A fund's principal market segment(s), such as large cap,
mid cap or small cap stocks, or growth or value stocks, may underperform other
market segments or the equity markets as a whole. Investments in smaller
companies and mid-size companies may involve greater risk and price volatility
than investments in larger, more mature companies.

Fixed-Income Securities Risk. Fixed-income securities are subject to interest
rate risk and credit quality risk. The market value of fixed-income securities
generally declines when interest rates rise, and an issuer of fixed-income
securities could default on its payment obligations.

Foreign Securities Risk. The risks of foreign securities include loss of value
as a result of: political or economic instability; nationalization,
expropriation or confiscatory taxation; changes in foreign exchange rates and
foreign exchange restrictions; settlement delays; and limited government
regulation (including less stringent reporting, accounting, and disclosure
standards than are required of U.S. companies).

High Yield Securities Risk. High yield fixed-income securities (commonly
referred to as "junk bonds") are subject to greater credit quality risk than
higher rated fixed-income securities and should be considered speculative.

Industry Concentration (Sector) Risk. A fund that concentrates investments in a
particular industry or group of industries (e.g., energy, real estate,
technology, financial services) has greater exposure than other funds to market,
economic and other factors affecting that industry or sector.

Portfolio Duration Risk. Portfolio duration is a measure of the expected life of
a fixed-income security and its sensitivity to changes in interest rates. The
longer a fund's average portfolio duration, the more sensitive the fund will be
to changes in interest rates.

Preferred Securities Risk. Preferred securities are junior subordinated
securities in a company's capital structure and therefore can be subject to
greater credit and liquidation risk. An issuer of preferred securities could
redeem the security prior to the stated maturity date and reduce the return of
the security.

Real Estate Investment Trusts ("REITs") Risk. A REIT could fail to qualify for
tax-free pass-through of income under the Internal Revenue Code, and fund
shareholders will indirectly bear their proportionate share of the expenses of
REITs in which the fund invests.

Real Estate Securities Risk. Real estate securities are subject to the risks
associated with direct ownership of real estate, including declines in value,
adverse economic conditions, increases in expenses, regulatory changes and
environmental problems.

Risk of Being an Underlying Fund. An underlying fund of a fund of funds may
experience relatively large redemptions or investments as the fund of funds
periodically reallocates or rebalances its assets. These transactions may cause
the underlying fund to sell portfolio securities to meet such redemptions, or to
invest cash from such investments, at times it would not otherwise do so, and
may as a result increase transaction costs and adversely affect underlying fund
performance.
Risk, Lose Money rr_RiskLoseMoney Many factors affect that value, and it is possible to lose money by investing in the Fund.
Risk, Not Insured Depository Institution rr_RiskNotInsuredDepositoryInstitution An investment in the Fund is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table, Heading rr_BarChartAndPerformanceTableHeading Performance
Performance, Narrative rr_PerformanceNarrativeTextBlock 
The following information provides an indication of the risks of investing in
the Fund. The bar chart shows the investment returns of the Fund's Class R-2
shares for each full calendar year of operations for 10 years (or, if shorter,
the life of the Fund). The table shows, for each share class of the Fund and for
the last one, five, and ten calendar year periods (or, if shorter, the life of
the Fund), how the Fund's average annual total returns compare to the returns of
one or more broad-based market indices. Past performance (before and after
taxes) is not necessarily an indication of how the Fund will perform in the
future. You may get updated performance information online at www.principal.com
or by calling 1-800-547-7754.

The R-1 Class shares were first sold on November 1, 2004. The other classes were
first sold on June 1, 2004. For periods prior to the date on which these classes
began operations, their returns are based on the performance of the Fund's
Institutional Class shares adjusted to reflect the fees and expenses of these
classes. The adjustments result in performance (for the periods prior to the
date these classes began operations) that is no higher than the historical
performance of the Institutional Class shares.

Performance of a blended index shows how the Fund's performance compares to an
index with similar investment objectives. Performance of the components of the
blended index are also shown. The weightings for the Preferreds Blended Index in
the Average Annual Total Returns table are 50% BofA Merrill Lynch Fixed Rate
Preferred Securities and 50% Barclays Capital U.S. Tier I Capital Securities
Index. The blended or custom index returns reflect the weightings as of the most
recent period for which fund returns are disclosed and do not reflect previous
weightings of the blended or custom index returns.
Performance, Information Illustrates Variability of Returns rr_PerformanceInformationIllustratesVariabilityOfReturns The following information provides an indication of the risks of investing in the Fund.
Performance, Availability Phone Number rr_PerformanceAvailabilityPhone 1-800-547-7754
Performance, Availability Website Address rr_PerformanceAvailabilityWebSiteAddress www.principal.com
Performance, Past Does Not Indicate Future rr_PerformancePastDoesNotIndicateFuture Past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future.
Bar Chart, Heading rr_BarChartHeading Total Returns as of December 31 each year (Class R-2 shares) [1]
Bar Chart, Closing rr_BarChartClosingTextBlock 
Highest return for a quarter during  Q2 '09 38.55%
the period of the bar chart above:

Lowest return for a quarter during   Q3 '08 -21.14%
the period of the bar chart above:
Index No Deduction for Fees, Expenses, Taxes rr_IndexNoDeductionForFeesExpensesTaxes reflects no deduction for fees, expenses, or taxes
Performance Table, Uses Highest Federal Rate rr_PerformanceTableUsesHighestFederalRate After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes.
Performance Table, Not Relevant to Tax Deferred rr_PerformanceTableNotRelevantToTaxDeferred Actual after-tax returns depend on an investor's tax situation and may differ from those shown. The after-tax returns shown are not relevant to investors who hold their Fund shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.
Performance Table, One Class of after Tax Shown rr_PerformanceTableOneClassOfAfterTaxShown After-tax returns are shown for Class R-1 shares only and would be different for Class R-2, R-3, R-4 and R-5 shares.
Performance Table, Closing rr_PerformanceTableClosingTextBlock 
After-tax returns are calculated using the historical highest individual federal
marginal income tax rates and do not reflect the impact of state and local
taxes. Actual after-tax returns depend on an investor's tax situation and may
differ from those shown. The after-tax returns shown are not relevant to
investors who hold their Fund shares through tax-deferred arrangements, such as
401(k) plans or individual retirement accounts. After-tax returns are shown for
Class R-1 shares only and would be different for Class R-2, R-3, R-4 and R-5
shares.
Average Annual Returns, Caption rr_AverageAnnualReturnCaption Average Annual Total Returns For the periods ended December 31, 2010
Preferred Securities Fund (Fourth Prospectus Summary) | Preferred Securities Fund | BofA Merrill Lynch Fixed Rate Preferred Securities Index
  
Risk Return [Abstract] rr_RiskReturnAbstract  
Average Annual Returns, Label rr_AverageAnnualReturnLabel BofA Merrill Lynch Fixed Rate Preferred Securities Index (reflects no deduction for fees, expenses, or taxes)
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 13.66%
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 (0.44%)
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 2.22%
Average Annual Returns, Inception Date Quaternary ck0000898745_AverageAnnualReturnInceptionDateQuaternary Nov. 01, 2004
Preferred Securities Fund (Fourth Prospectus Summary) | Preferred Securities Fund | Barclays Capital U.S. Tier I Capital Securities Index
  
Risk Return [Abstract] rr_RiskReturnAbstract  
Average Annual Returns, Label rr_AverageAnnualReturnLabel Barclays Capital U.S. Tier I Capital Securities Index (reflects no deduction for fees, expenses, or taxes)
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 17.04%
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 2.83%
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 5.51%
Average Annual Returns, Inception Date Quaternary ck0000898745_AverageAnnualReturnInceptionDateQuaternary Nov. 01, 2004
Preferred Securities Fund (Fourth Prospectus Summary) | Preferred Securities Fund | Preferreds Blended Index
  
Risk Return [Abstract] rr_RiskReturnAbstract  
Average Annual Returns, Label rr_AverageAnnualReturnLabel Preferreds Blended Index (reflects no deduction for fees, expenses, or taxes)
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 14.87%
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 1.02%
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 3.57%
Average Annual Returns, Inception Date Quaternary ck0000898745_AverageAnnualReturnInceptionDateQuaternary Nov. 01, 2004
Preferred Securities Fund (Fourth Prospectus Summary) | Preferred Securities Fund | Class R-1
  
Risk Return [Abstract] rr_RiskReturnAbstract  
Management Fees rr_ManagementFeesOverAssets 0.71%
Distribution and/or Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets 0.35%
Other Expenses rr_OtherExpensesOverAssets 0.54%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 1.60%
Expense Example, With Redemption, 1 Year rr_ExpenseExampleYear01 163
Expense Example, With Redemption, 3 Years rr_ExpenseExampleYear03 505
Expense Example, With Redemption, 5 Years rr_ExpenseExampleYear05 871
Expense Example, With Redemption, 10 Years rr_ExpenseExampleYear10 1,900
Average Annual Returns, Label rr_AverageAnnualReturnLabel Class R-1 Return Before Taxes
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 15.45%
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 4.56%
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 4.71%
Average Annual Returns, Inception Date rr_AverageAnnualReturnInceptionDate Nov. 01, 2004
Preferred Securities Fund (Fourth Prospectus Summary) | Preferred Securities Fund | Class R-1 | After Taxes on Distributions
  
Risk Return [Abstract] rr_RiskReturnAbstract  
Average Annual Returns, Label rr_AverageAnnualReturnLabel Class R-1 Return After Taxes on Distributions
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 13.24%
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 2.69%
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 3.03%
Average Annual Returns, Inception Date rr_AverageAnnualReturnInceptionDate Nov. 01, 2004
Preferred Securities Fund (Fourth Prospectus Summary) | Preferred Securities Fund | Class R-1 | After Taxes on Distributions and Sales
  
Risk Return [Abstract] rr_RiskReturnAbstract  
Average Annual Returns, Label rr_AverageAnnualReturnLabel Class R-1 Return After Taxes on Distribution and Sale of Fund Shares
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 10.27%
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 2.82%
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 3.07%
Average Annual Returns, Inception Date rr_AverageAnnualReturnInceptionDate Nov. 01, 2004
Preferred Securities Fund (Fourth Prospectus Summary) | Preferred Securities Fund | Class R-2
  
Risk Return [Abstract] rr_RiskReturnAbstract  
Management Fees rr_ManagementFeesOverAssets 0.71%
Distribution and/or Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets 0.30%
Other Expenses rr_OtherExpensesOverAssets 0.46%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 1.47%
Expense Example, With Redemption, 1 Year rr_ExpenseExampleYear01 150
Expense Example, With Redemption, 3 Years rr_ExpenseExampleYear03 465
Expense Example, With Redemption, 5 Years rr_ExpenseExampleYear05 803
Expense Example, With Redemption, 10 Years rr_ExpenseExampleYear10 1,757
Annual Return 2003 rr_AnnualReturn2003 9.89%
Annual Return 2004 rr_AnnualReturn2004 3.61%
Annual Return 2005 rr_AnnualReturn2005 0.78%
Annual Return 2006 rr_AnnualReturn2006 6.27%
Annual Return 2007 rr_AnnualReturn2007 (8.80%)
Annual Return 2008 rr_AnnualReturn2008 (22.77%)
Annual Return 2009 rr_AnnualReturn2009 45.43%
Annual Return 2010 rr_AnnualReturn2010 15.55%
Year to Date Return, Label rr_YearToDateReturnLabel year-to-date return
Bar Chart, Year to Date Return, Date rr_BarChartYearToDateReturnDate Sep. 30, 2011
Bar Chart, Year to Date Return rr_BarChartYearToDateReturn (1.02%)
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Highest return for a quarter
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Jun. 30, 2009
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 38.55%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Lowest return for a quarter
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Sep. 30, 2008
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (21.14%)
Average Annual Returns, Label rr_AverageAnnualReturnLabel Class R-2 Return Before Taxes
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 15.55%
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 4.69%
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 4.85%
Average Annual Returns, Inception Date rr_AverageAnnualReturnInceptionDate Jun. 01, 2004
Preferred Securities Fund (Fourth Prospectus Summary) | Preferred Securities Fund | Class R-3
  
Risk Return [Abstract] rr_RiskReturnAbstract  
Management Fees rr_ManagementFeesOverAssets 0.71%
Distribution and/or Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets 0.25%
Other Expenses rr_OtherExpensesOverAssets 0.33%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 1.29%
Expense Example, With Redemption, 1 Year rr_ExpenseExampleYear01 131
Expense Example, With Redemption, 3 Years rr_ExpenseExampleYear03 409
Expense Example, With Redemption, 5 Years rr_ExpenseExampleYear05 708
Expense Example, With Redemption, 10 Years rr_ExpenseExampleYear10 1,556
Average Annual Returns, Label rr_AverageAnnualReturnLabel Class R-3 Return Before Taxes
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 15.82%
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 4.90%
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 5.05%
Average Annual Returns, Inception Date rr_AverageAnnualReturnInceptionDate Jun. 01, 2004
Preferred Securities Fund (Fourth Prospectus Summary) | Preferred Securities Fund | Class R-4
  
Risk Return [Abstract] rr_RiskReturnAbstract  
Management Fees rr_ManagementFeesOverAssets 0.71%
Distribution and/or Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets 0.10%
Other Expenses rr_OtherExpensesOverAssets 0.29%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 1.10%
Expense Example, With Redemption, 1 Year rr_ExpenseExampleYear01 112
Expense Example, With Redemption, 3 Years rr_ExpenseExampleYear03 350
Expense Example, With Redemption, 5 Years rr_ExpenseExampleYear05 606
Expense Example, With Redemption, 10 Years rr_ExpenseExampleYear10 1,340
Average Annual Returns, Label rr_AverageAnnualReturnLabel Class R-4 Return Before Taxes
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 16.05%
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 5.06%
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 5.22%
Average Annual Returns, Inception Date rr_AverageAnnualReturnInceptionDate Jun. 01, 2004
Preferred Securities Fund (Fourth Prospectus Summary) | Preferred Securities Fund | Class R-5
  
Risk Return [Abstract] rr_RiskReturnAbstract  
Management Fees rr_ManagementFeesOverAssets 0.71%
Distribution and/or Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets   
Other Expenses rr_OtherExpensesOverAssets 0.27%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.98%
Expense Example, With Redemption, 1 Year rr_ExpenseExampleYear01 100
Expense Example, With Redemption, 3 Years rr_ExpenseExampleYear03 312
Expense Example, With Redemption, 5 Years rr_ExpenseExampleYear05 542
Expense Example, With Redemption, 10 Years rr_ExpenseExampleYear10 1,201
Average Annual Returns, Label rr_AverageAnnualReturnLabel Class R-5 Return Before Taxes
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 16.12%
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 5.20%
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 5.36%
Average Annual Returns, Inception Date rr_AverageAnnualReturnInceptionDate Jun. 01, 2004
[1] The year-to-date return as of September 30, 2011 was -1.02% for Class R-2 shares.