Investments (Details 6) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Fixed maturities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	$ 8,575.7	$ 1,676.9
Less than twelve months, Gross unrealized losses	322.7	19.8
Greater than or equal to twelve months, Fair value	2,264.3	3,320.0
Greater than or equal to twelve months, Gross unrealized losses	561.0	851.3
Total, Fair value	10,840.0	4,996.9
Total, Gross unrealized losses	883.7	871.1
U.S. government and agencies
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	399.5	115.4
Less than twelve months, Gross unrealized losses	36.4	0.3
Total, Fair value	399.5	115.4
Total, Gross unrealized losses	36.4	0.3
Non-U.S. governments
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	79.6	17.3
Less than twelve months, Gross unrealized losses	3.7	0.2
Greater than or equal to twelve months, Fair value	4.3	13.4
Greater than or equal to twelve months, Gross unrealized losses	0.1	0.7
Total, Fair value	83.9	30.7
Total, Gross unrealized losses	3.8	0.9
States and political subdivisions
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	1,221.6	235.3
Less than twelve months, Gross unrealized losses	61.2	2.1
Greater than or equal to twelve months, Fair value	45.0	8.8
Greater than or equal to twelve months, Gross unrealized losses	9.4	0.6
Total, Fair value	1,266.6	244.1
Total, Gross unrealized losses	70.6	2.7
Corporate debt securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	4,189.8	831.8
Less than twelve months, Gross unrealized losses	163.3	10.6
Greater than or equal to twelve months, Fair value	1,369.5	1,961.7
Greater than or equal to twelve months, Gross unrealized losses	220.5	309.3
Total, Fair value	5,559.3	2,793.5
Total, Gross unrealized losses	383.8	319.9
Residential mortgage-backed pass-through securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	795.6	70.4
Less than twelve months, Gross unrealized losses	27.8	0.3
Greater than or equal to twelve months, Fair value	3.0	2.4
Greater than or equal to twelve months, Gross unrealized losses	0.1	0.1
Total, Fair value	798.6	72.8
Total, Gross unrealized losses	27.9	0.4
Commercial mortgage-backed securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	563.2	98.9
Less than twelve months, Gross unrealized losses	12.9	3.3
Greater than or equal to twelve months, Fair value	543.2	785.0
Greater than or equal to twelve months, Gross unrealized losses	268.4	435.8
Total, Fair value	1,106.4	883.9
Total, Gross unrealized losses	281.3	439.1
Collateralized debt obligations
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	53.2	72.2
Less than twelve months, Gross unrealized losses	0.6	1.0
Greater than or equal to twelve months, Fair value	47.3	133.8
Greater than or equal to twelve months, Gross unrealized losses	32.9	55.6
Total, Fair value	100.5	206.0
Total, Gross unrealized losses	33.5	56.6
Other debt obligations
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	1,273.2	235.6
Less than twelve months, Gross unrealized losses	16.8	2.0
Greater than or equal to twelve months, Fair value	252.0	414.9
Greater than or equal to twelve months, Gross unrealized losses	29.6	49.2
Total, Fair value	1,525.2	650.5
Total, Gross unrealized losses	46.4	51.2
Equity securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	27.3	5.8
Less than twelve months, Gross unrealized losses	0.1	0.1
Greater than or equal to twelve months, Fair value	48.5	52.9
Greater than or equal to twelve months, Gross unrealized losses	12.8	8.4
Total, Fair value	75.8	58.7
Total, Gross unrealized losses	12.9	8.5
Principal Life Insurance Company | Fixed maturities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	8,186.9	1,382.1
Less than twelve months, Gross unrealized losses	305.8	16.2
Greater than or equal to twelve months, Fair value	2,010.9	3,037.3
Greater than or equal to twelve months, Gross unrealized losses	522.0	809.5
Total, Fair value	10,197.8	4,419.4
Total, Gross unrealized losses	827.8	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	1,102	224
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Twelve Months or Longer	368	488
Available-for-sale Securities in Unrealized Loss Position, Aggregate Losses On Investment Grade Investments, Less Than Twelve Months	287.1	13.3
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Percent Investment Grade (as a percent)	84.00%	71.00%
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Average Price (percent of carrying value to amortized cost)	92	84
Available-for-sale Securities in Unrealized Loss Positions, Percent Investment Grade, Less Than Twelve Months (as a percent)	91.00%	89.00%
Available-for-sale Securities in Unrealized Loss Positions, Average Price, Less Than Twelve Months (percent of carrying value to amortized cost)	96	99
Available-for-sale Securities in Unrealized Loss Positions, Average Price, Twelve Months or Longer (percent of carrying value to amortized cost)	79	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	181.6	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)	86	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	$ 268.4	$ 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)	67	64