Investments (Details 6) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Fixed maturities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	$ 1,343.9	$ 5,021.3
Less than twelve months, Gross unrealized losses	29.3	215.7
Greater than or equal to twelve months, Fair value	4,081.3	4,031.0
Greater than or equal to twelve months, Gross unrealized losses	1,004.0	1,308.3
Total, Fair value	5,425.2	9,052.3
Total, Gross unrealized losses	1,033.3	1,524.0
U.S. government and agencies
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	16.0
Total, Fair value	16.0
Non-U.S. governments
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	35.0	68.5
Less than twelve months, Gross unrealized losses	1.6	1.4
Greater than or equal to twelve months, Fair value	0.4	0.3
Total, Fair value	35.4	68.8
Total, Gross unrealized losses	1.6	1.4
States and political subdivisions
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	196.1	5.7
Less than twelve months, Gross unrealized losses	1.9	0.1
Greater than or equal to twelve months, Fair value	13.8	51.7
Greater than or equal to twelve months, Gross unrealized losses	0.9	5.4
Total, Fair value	209.9	57.4
Total, Gross unrealized losses	2.8	5.5
Corporate debt securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	737.0	3,445.6
Less than twelve months, Gross unrealized losses	18.2	140.9
Greater than or equal to twelve months, Fair value	2,573.4	2,403.9
Greater than or equal to twelve months, Gross unrealized losses	391.9	578.1
Total, Fair value	3,310.4	5,849.5
Total, Gross unrealized losses	410.1	719.0
Residential mortgage-backed pass-through securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	6.5	77.8
Less than twelve months, Gross unrealized losses		0.5
Greater than or equal to twelve months, Fair value	2.4	3.7
Greater than or equal to twelve months, Gross unrealized losses	0.1	0.2
Total, Fair value	8.9	81.5
Total, Gross unrealized losses	0.1	0.7
Commercial mortgage-backed securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	113.5	608.4
Less than twelve months, Gross unrealized losses	4.0	57.3
Greater than or equal to twelve months, Fair value	840.6	858.9
Greater than or equal to twelve months, Gross unrealized losses	485.2	540.3
Total, Fair value	954.1	1,467.3
Total, Gross unrealized losses	489.2	597.6
Collateralized debt obligations
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	69.9	107.2
Less than twelve months, Gross unrealized losses	1.9	2.5
Greater than or equal to twelve months, Fair value	150.7	204.4
Greater than or equal to twelve months, Gross unrealized losses	61.9	60.3
Total, Fair value	220.6	311.6
Total, Gross unrealized losses	63.8	62.8
Other debt obligations
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	169.9	708.1
Less than twelve months, Gross unrealized losses	1.7	13.0
Greater than or equal to twelve months, Fair value	500.0	508.1
Greater than or equal to twelve months, Gross unrealized losses	64.0	124.0
Total, Fair value	669.9	1,216.2
Total, Gross unrealized losses	65.7	137.0
Equity securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	5.0	14.3
Less than twelve months, Gross unrealized losses	0.1	3.2
Greater than or equal to twelve months, Fair value	51.5	15.6
Greater than or equal to twelve months, Gross unrealized losses	9.9	3.3
Total, Fair value	56.5	29.9
Total, Gross unrealized losses	10.0	6.5
Principal Life Insurance Company | Fixed maturities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	1,071.2	4,573.6
Less than twelve months, Gross unrealized losses	17.4	198.7
Greater than or equal to twelve months, Fair value	3,893.2	3,967.1
Greater than or equal to twelve months, Gross unrealized losses	961.2	1,271.6
Total, Fair value	4,964.4	8,540.7
Total, Gross unrealized losses	978.6	1,470.3
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	183	477
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Twelve Months or Longer	554	628
Available-for-sale Securities in Unrealized Loss Position, Aggregate Losses On Investment Grade Investments, Less Than Twelve Months	13.8	128.5
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Percent Investment Grade (as a percent)	70.00%	76.00%
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Average Price (percent of carrying value to amortized cost)	84	85
Available-for-sale Securities in Unrealized Loss Positions, Percent Investment Grade, Less Than Twelve Months (as a percent)	87.00%	86.00%
Available-for-sale Securities in Unrealized Loss Positions, Average Price, Less Than Twelve Months (percent of carrying value to amortized cost)	98	96
Available-for-sale Securities in Unrealized Loss Positions, Average Price, Twelve Months or Longer (percent of carrying value to amortized cost)	80	76
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	349.0	541.4
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)	87	81
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	$ 485.2	$ 540.3
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)	63	61