Investments (Details 6) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Fixed maturities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	$ 2,796.6	$ 1,676.9
Less than twelve months, Gross unrealized losses	40.0	19.8
Greater than or equal to twelve months, Fair value	2,697.5	3,320.0
Greater than or equal to twelve months, Gross unrealized losses	719.6	851.3
Total, Fair value	5,494.1	4,996.9
Total, Gross unrealized losses	759.6	871.1
U.S. government and agencies
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	233.0	115.4
Less than twelve months, Gross unrealized losses	6.1	0.3
Total, Fair value	233.0	115.4
Total, Gross unrealized losses	6.1	0.3
Non-U.S. governments
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	30.0	17.3
Less than twelve months, Gross unrealized losses	1.3	0.2
Greater than or equal to twelve months, Fair value	11.6	13.4
Greater than or equal to twelve months, Gross unrealized losses	1.8	0.7
Total, Fair value	41.6	30.7
Total, Gross unrealized losses	3.1	0.9
States and political subdivisions
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	342.9	235.3
Less than twelve months, Gross unrealized losses	3.3	2.1
Greater than or equal to twelve months, Fair value	8.7	8.8
Greater than or equal to twelve months, Gross unrealized losses	0.5	0.6
Total, Fair value	351.6	244.1
Total, Gross unrealized losses	3.8	2.7
Corporate debt securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	1,318.1	831.8
Less than twelve months, Gross unrealized losses	21.1	10.6
Greater than or equal to twelve months, Fair value	1,619.7	1,961.7
Greater than or equal to twelve months, Gross unrealized losses	268.2	309.3
Total, Fair value	2,937.8	2,793.5
Total, Gross unrealized losses	289.3	319.9
Residential mortgage-backed pass-through securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	349.6	70.4
Less than twelve months, Gross unrealized losses	3.8	0.3
Greater than or equal to twelve months, Fair value	2.1	2.4
Greater than or equal to twelve months, Gross unrealized losses	0.1	0.1
Total, Fair value	351.7	72.8
Total, Gross unrealized losses	3.9	0.4
Commercial mortgage-backed securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	96.3	98.9
Less than twelve months, Gross unrealized losses	1.4	3.3
Greater than or equal to twelve months, Fair value	687.1	785.0
Greater than or equal to twelve months, Gross unrealized losses	370.5	435.8
Total, Fair value	783.4	883.9
Total, Gross unrealized losses	371.9	439.1
Collateralized debt obligations
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	30.2	72.2
Less than twelve months, Gross unrealized losses	0.1	1.0
Greater than or equal to twelve months, Fair value	92.4	133.8
Greater than or equal to twelve months, Gross unrealized losses	46.2	55.6
Total, Fair value	122.6	206.0
Total, Gross unrealized losses	46.3	56.6
Other debt obligations
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	396.5	235.6
Less than twelve months, Gross unrealized losses	2.9	2.0
Greater than or equal to twelve months, Fair value	275.9	414.9
Greater than or equal to twelve months, Gross unrealized losses	32.3	49.2
Total, Fair value	672.4	650.5
Total, Gross unrealized losses	35.2	51.2
Equity securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	5.0	5.8
Less than twelve months, Gross unrealized losses		0.1
Greater than or equal to twelve months, Fair value	55.4	52.9
Greater than or equal to twelve months, Gross unrealized losses	6.0	8.4
Total, Fair value	60.4	58.7
Total, Gross unrealized losses	6.0	8.5
Principal Life Insurance Company | Fixed maturities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	2,413.1	1,382.1
Less than twelve months, Gross unrealized losses	30.0	16.2
Greater than or equal to twelve months, Fair value	2,396.5	3,037.3
Greater than or equal to twelve months, Gross unrealized losses	676.5	809.5
Total, Fair value	4,809.6	4,419.4
Total, Gross unrealized losses	706.5	825.7
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	309	224
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Twelve Months or Longer	417	488
Available-for-sale Securities in Unrealized Loss Position, Aggregate Losses On Investment Grade Investments, Less Than Twelve Months	27.4	13.3
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Percent Investment Grade (as a percent)	76.00%	71.00%
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Average Price (percent of carrying value to amortized cost)	87	84
Available-for-sale Securities in Unrealized Loss Positions, Percent Investment Grade, Less Than Twelve Months (as a percent)	93.00%	89.00%
Available-for-sale Securities in Unrealized Loss Positions, Average Price, Less Than Twelve Months (percent of carrying value to amortized cost)	99	99
Available-for-sale Securities in Unrealized Loss Positions, Average Price, Twelve Months or Longer (percent of carrying value to amortized cost)	78	79
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	227.0	268.1
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)	85	86
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	$ 370.4	$ 435.8
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)	65	64