Label	Element	Value
Risk Return [Abstract]	rr_RiskReturnAbstract
ProspectusDate	rr_ProspectusDate	Jul. 17, 2012
Government & High Quality Bond Fund f/k/a Mortgage Securities Fund (Fourth Prospectus Summary) | Government & High Quality Bond Fund f/k/a Mortgage Securities Fund
Risk Return [Abstract]	rr_RiskReturnAbstract
Risk/Return, Heading	rr_RiskReturnHeading	Government & High Quality Bond Fund
Investment Objective, Heading	rr_ObjectiveHeading	Objective:
investment Objective, Primary	rr_ObjectivePrimaryTextBlock	The Fund seeks to provide a high level of current income consistent with safety and liquidity.
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 portfolio turnover rate was 104.7% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	104.70%
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, plus borrowings for investment purposes, in securities issued by the U.S. government, its agencies or instrumentalities or securities that are rated AAA by S&P, AAA by Fitch, or Aaa by Moody's, including but not limited to mortgage securities such as agency and non-agency collateralized mortgage obligations, and other obligations that are secured by mortgages or mortgage-backed securities. Under normal circumstances, the Fund maintains an average portfolio duration between one and 4.5 years. The Fund may also invest in mortgage-backed securities that are not issued by the U.S. government, its agencies or instrumentalities or rated AAA by S&P, AAA by Fitch, or Aaa by Moody's, including collateralized mortgage obligations, and in other obligations that are secured by mortgages or mortgage-backed securities. The Fund may actively trade portfolio securities.
Risk, Heading	rr_RiskHeading	Principal Risks
Risk, Narrative	rr_RiskNarrativeTextBlock	The Fund may be an appropriate investment for investors seeking diversification by investing in a fixed-income mutual fund. 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:    Active Trading Risk. Actively trading portfolio securities may accelerate realization of taxable gains and losses, lower fund performance and may result in high portfolio turnover rates and increased brokerage costs. 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. 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. Prepayment Risk. Unscheduled prepayments on mortgage-backed and asset-backed securities may have to be reinvested at lower rates. A reduction in prepayments may increase the effective maturities of these securities, exposing them to the risk of decline in market value over time (extension risk). 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. A fund is subject to the risk of being an underlying fund to the extent that a fund of funds invests in the 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. U.S. Government Securities Risk. Yields available from U.S. government securities are generally lower than yields from many other fixed-income securities. U.S. Government Sponsored Securities Risk. Securities issued by U.S. government-sponsored or -chartered enterprises such as the Federal Home Loan Mortgage Corporation, the Federal National Mortgage Association, and the Federal Home Loan Banks are not issued or guaranteed by the U.S. Treasury.
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 Institutional Class shares for each full calendar year of operations for 10 years (or, if shorter, the life of the Fund). The table shows, for Institutional Class shares 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 1-800-222-5852. The Fund commenced operations after succeeding to the operations of another fund on January 12, 2007. Performance for periods prior to that date is based on the performance of the predecessor fund which commenced operations on May 4, 1984.
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	1-800-222-5852
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 (Institutional Class shares)
Bar Chart, Closing	rr_BarChartClosingTextBlock	Highest return for a quarter during the period of the bar  Q2 '02     3.50% chart above: Lowest return for a quarter during the period of the bar   Q2 '04    -1.05% 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, 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.
Average Annual Returns, Caption	rr_AverageAnnualReturnCaption	Average Annual Total Returns For the periods ended December 31, 2011
Government & High Quality Bond Fund f/k/a Mortgage Securities Fund (Fourth Prospectus Summary) | Government & High Quality Bond Fund f/k/a Mortgage Securities Fund | Barclays Capital MBS Fixed Rate Index
Risk Return [Abstract]	rr_RiskReturnAbstract
Average Annual Returns, Label	rr_AverageAnnualReturnLabel	Barclays Capital MBS Fixed Rate Index (reflects no deduction for fees, expenses or taxes)
Average Annual Returns, 1 Year	rr_AverageAnnualReturnYear01	6.32%
Average Annual Returns, 5 Years	rr_AverageAnnualReturnYear05	6.61%
Average Annual Returns, 10 Years	rr_AverageAnnualReturnYear10	5.72%
Government & High Quality Bond Fund f/k/a Mortgage Securities Fund (Fourth Prospectus Summary) | Government & High Quality Bond Fund f/k/a Mortgage Securities Fund | Institutional Class
Risk Return [Abstract]	rr_RiskReturnAbstract
Management Fees	rr_ManagementFeesOverAssets	0.50%
Other Expenses	rr_OtherExpensesOverAssets	0.01%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	0.51%
Expense Example, With Redemption, 1 Year	rr_ExpenseExampleYear01	52
Expense Example, With Redemption, 3 Years	rr_ExpenseExampleYear03	164
Expense Example, With Redemption, 5 Years	rr_ExpenseExampleYear05	285
Expense Example, With Redemption, 10 Years	rr_ExpenseExampleYear10	640
Annual Return 2002	rr_AnnualReturn2002	8.87%
Annual Return 2003	rr_AnnualReturn2003	2.21%
Annual Return 2004	rr_AnnualReturn2004	3.95%
Annual Return 2005	rr_AnnualReturn2005	2.31%
Annual Return 2006	rr_AnnualReturn2006	4.67%
Annual Return 2007	rr_AnnualReturn2007	6.34%
Annual Return 2008	rr_AnnualReturn2008	4.04%
Annual Return 2009	rr_AnnualReturn2009	8.07%
Annual Return 2010	rr_AnnualReturn2010	6.23%
Annual Return 2011	rr_AnnualReturn2011	6.14%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Highest return for a quarter
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Jun. 30, 2002
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	3.50%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Lowest return for a quarter
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Jun. 30, 2004
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(1.05%)
Average Annual Returns, Label	rr_AverageAnnualReturnLabel	Institutional Class Return Before Taxes
Average Annual Returns, 1 Year	rr_AverageAnnualReturnYear01	6.14%
Average Annual Returns, 5 Years	rr_AverageAnnualReturnYear05	6.16%
Average Annual Returns, 10 Years	rr_AverageAnnualReturnYear10	5.26%
Government & High Quality Bond Fund f/k/a Mortgage Securities Fund (Fourth Prospectus Summary) | Government & High Quality Bond Fund f/k/a Mortgage Securities Fund | Institutional Class | After Taxes on Distributions
Risk Return [Abstract]	rr_RiskReturnAbstract
Average Annual Returns, Label	rr_AverageAnnualReturnLabel	Institutional Class Return After Taxes on Distributions
Average Annual Returns, 1 Year	rr_AverageAnnualReturnYear01	4.70%
Average Annual Returns, 5 Years	rr_AverageAnnualReturnYear05	4.47%
Average Annual Returns, 10 Years	rr_AverageAnnualReturnYear10	3.50%
Government & High Quality Bond Fund f/k/a Mortgage Securities Fund (Fourth Prospectus Summary) | Government & High Quality Bond Fund f/k/a Mortgage Securities Fund | Institutional Class | After Taxes on Distributions and Sales
Risk Return [Abstract]	rr_RiskReturnAbstract
Average Annual Returns, Label	rr_AverageAnnualReturnLabel	Institutional Class Return After Taxes on Distribution and Sale of Fund Shares
Average Annual Returns, 1 Year	rr_AverageAnnualReturnYear01	3.97%
Average Annual Returns, 5 Years	rr_AverageAnnualReturnYear05	4.27%
Average Annual Returns, 10 Years	rr_AverageAnnualReturnYear10	3.44%