Investments (Details 7) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Fixed maturities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Carrying value	$ 5,021.3	$ 4,597.7
Less than twelve months, Gross unrealized losses	215.6	107.4
Greater than or equal to twelve months, Carrying value	4,031.0	5,990.9
Greater than or equal to twelve months, Gross unrealized losses	1,308.3	1,381.4
Total, Carrying value	9,052.3	10,588.6
Total, Gross unrealized losses	1,523.9	1,488.8
U.S. government and agencies
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Carrying value		224.5
Less than twelve months, Gross unrealized losses		0.2
Total, Carrying value		224.5
Total, Gross unrealized losses		0.2
Non-U.S. governments
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Carrying value	68.5	7.9
Less than twelve months, Gross unrealized losses	1.4
Greater than or equal to twelve months, Carrying value	0.3
Total, Carrying value	68.8	7.9
Total, Gross unrealized losses	1.4
States and political subdivisions
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Carrying value	5.7	771.0
Less than twelve months, Gross unrealized losses	0.1	18.4
Greater than or equal to twelve months, Carrying value	51.7	44.2
Greater than or equal to twelve months, Gross unrealized losses	5.4	4.9
Total, Carrying value	57.4	815.2
Total, Gross unrealized losses	5.5	23.3
Corporate debt securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Carrying value	3,445.6	2,457.4
Less than twelve months, Gross unrealized losses	140.8	69.1
Greater than or equal to twelve months, Carrying value	2,403.9	3,948.9
Greater than or equal to twelve months, Gross unrealized losses	578.1	475.9
Total, Carrying value	5,849.5	6,406.3
Total, Gross unrealized losses	718.9	545.0
Residential mortgage-backed pass-through securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Carrying value	77.8	384.9
Less than twelve months, Gross unrealized losses	0.5	5.9
Greater than or equal to twelve months, Carrying value	3.7
Greater than or equal to twelve months, Gross unrealized losses	0.2
Total, Carrying value	81.5	384.9
Total, Gross unrealized losses	0.7	5.9
Commercial mortgage-backed securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Carrying value	608.4	340.1
Less than twelve months, Gross unrealized losses	57.3	4.9
Greater than or equal to twelve months, Carrying value	858.9	1,186.4
Greater than or equal to twelve months, Gross unrealized losses	540.3	695.8
Total, Carrying value	1,467.3	1,526.5
Total, Gross unrealized losses	597.6	700.7
Collateralized debt obligations
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Carrying value	107.2	10.4
Less than twelve months, Gross unrealized losses	2.5	0.5
Greater than or equal to twelve months, Carrying value	204.4	233.0
Greater than or equal to twelve months, Gross unrealized losses	60.3	88.7
Total, Carrying value	311.6	243.4
Total, Gross unrealized losses	62.8	89.2
Other debt obligations
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Carrying value	708.1	401.5
Less than twelve months, Gross unrealized losses	13.0	8.4
Greater than or equal to twelve months, Carrying value	508.1	578.4
Greater than or equal to twelve months, Gross unrealized losses	124.0	116.1
Total, Carrying value	1,216.2	979.9
Total, Gross unrealized losses	137.0	124.5
Equity securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Carrying value	14.3	47.3
Less than twelve months, Gross unrealized losses	3.2	7.2
Greater than or equal to twelve months, Carrying value	15.6	77.0
Greater than or equal to twelve months, Gross unrealized losses	3.3	11.0
Total, Carrying value	29.9	124.3
Total, Gross unrealized losses	6.5	18.2
Principal Life Insurance Company | Fixed maturities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Carrying value	4,573.6	4,112.3
Less than twelve months, Gross unrealized losses	198.7	95.7
Greater than or equal to twelve months, Carrying value	3,967.1	5,801.9
Greater than or equal to twelve months, Gross unrealized losses	1,271.6	1,349.6
Total, Carrying value	8,540.7	9,914.2
Total, Gross unrealized losses	1,470.3	1,445.3
Available-for-sale Securities, Continuous Unrealized Loss Position, Qualitative Disclosure
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less Than Twelve Months	477	534
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Twelve Months or Longer	628	773
Available-for-sale Securities in Unrealized Loss Position, Aggregate Losses On Investment Grade Investments, Less Than Twelve Months	128.5	88.7
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Percent Investment Grade (as a percent)	76.00%	77.00%
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Average Price (percent of carrying value to amortized cost)	85	87
Available-for-sale Securities in Unrealized Loss Positions, Percent Investment Grade, Less Than Twelve Months (as a percent)	86.00%	94.00%
Available-for-sale Securities in Unrealized Loss Positions, Average Price, Less Than Twelve Months (percent of carrying value to amortized cost)	96	98
Available-for-sale Securities in Unrealized Loss Positions, Average Price, Twelve Months or Longer (percent of carrying value to amortized cost)	76	81
Principal Life Insurance Company | Corporate debt securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Greater than or equal to twelve months, Gross unrealized losses	541.4	444.1
Available-for-sale Securities, Continuous Unrealized Loss Position, Qualitative Disclosure
Available-for-sale Securities in Unrealized Loss Positions, Average Price, Twelve Months or Longer (percent of carrying value to amortized cost)	81	89
Principal Life Insurance Company | Commercial mortgage-backed securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Greater than or equal to twelve months, Gross unrealized losses	$ 540.3	$ 695.8
Available-for-sale Securities, Continuous Unrealized Loss Position, Qualitative Disclosure
Available-for-sale Securities in Unrealized Loss Positions, Average Price, Twelve Months or Longer (percent of carrying value to amortized cost)	61	63