Debt (Details 4) (USD $) In Millions, unless otherwise specified			12 Months Ended				1 Months Ended		1 Months Ended
	Dec. 31, 2011 8% surplus notes payable, due 2044	Dec. 31, 2010 8% surplus notes payable, due 2044	Dec. 31, 2011 8% surplus notes payable, due 2044 Principal Life Insurance Company Only Y	Dec. 31, 2010 8% surplus notes payable, due 2044 Principal Life Insurance Company Only	Dec. 31, 2009 8% surplus notes payable, due 2044 Principal Life Insurance Company Only	Mar. 10, 1994 8% surplus notes payable, due 2044 Principal Life Insurance Company Only	Nov. 30, 2010 3.76% notes payable, due 2015 Principal International de Chile S.A.	Nov. 03, 2010 3.76% notes payable, due 2015 Principal International de Chile S.A.	Nov. 30, 2010 3.31% notes payable, due 2011 and 3.63% notes payable due 2011 Principal International de Chile S.A.
Long-Term Debt
Long-term debt issued						$ 100.0		$ 98.9
Long-term debt interest rate (as a percent)	8.00%	8.00%				8.00%
Monetary unit used for denominating Chilean debt							Unidades de Formento ("UF")
Spread on UF interest rate basis for Chilean debt (as a percent)								3.76%
Long-term debt consolidated and modified through a new borrowing agreement									93.9
Interest expense			$ 8.0	$ 8.0	$ 8.0
Redemption price of par on or after March 1, 2014 (as a percent)			102.30%
Redemption price premium (as a percent)			2.30%
Period for redemption price premium (in years)			10
Redemption price of principal amount plus interest accrued on or after March 1, 2024 (as a percent)			100.00%