Investments (Details 6) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Fixed maturities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	$ 9,081.3	$ 1,676.9
Less than twelve months, Gross unrealized losses	305.4	19.8
Greater than or equal to twelve months, Fair value	2,383.6	3,320.0
Greater than or equal to twelve months, Gross unrealized losses	628.9	851.3
Total, Fair value	11,464.9	4,996.9
Total, Gross unrealized losses	934.3	871.1
U.S. government and agencies
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	415.0	115.4
Less than twelve months, Gross unrealized losses	26.8	0.3
Total, Fair value	415.0	115.4
Total, Gross unrealized losses	26.8	0.3
Non-U.S. governments
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	95.7	17.3
Less than twelve months, Gross unrealized losses	4.1	0.2
Greater than or equal to twelve months, Fair value	17.5	13.4
Greater than or equal to twelve months, Gross unrealized losses	3.1	0.7
Total, Fair value	113.2	30.7
Total, Gross unrealized losses	7.2	0.9
States and political subdivisions
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	1,195.5	235.3
Less than twelve months, Gross unrealized losses	57.0	2.1
Greater than or equal to twelve months, Fair value	16.0	8.8
Greater than or equal to twelve months, Gross unrealized losses	2.4	0.6
Total, Fair value	1,211.5	244.1
Total, Gross unrealized losses	59.4	2.7
Corporate debt securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	4,359.6	831.8
Less than twelve months, Gross unrealized losses	154.4	10.6
Greater than or equal to twelve months, Fair value	1,465.3	1,961.7
Greater than or equal to twelve months, Gross unrealized losses	244.0	309.3
Total, Fair value	5,824.9	2,793.5
Total, Gross unrealized losses	398.4	319.9
Residential mortgage-backed pass-through securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	923.9	70.4
Less than twelve months, Gross unrealized losses	28.4	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	926.0	72.8
Total, Gross unrealized losses	28.5	0.4
Commercial mortgage-backed securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	611.3	98.9
Less than twelve months, Gross unrealized losses	14.3	3.3
Greater than or equal to twelve months, Fair value	569.1	785.0
Greater than or equal to twelve months, Gross unrealized losses	311.9	435.8
Total, Fair value	1,180.4	883.9
Total, Gross unrealized losses	326.2	439.1
Collateralized debt obligations
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	58.9	72.2
Less than twelve months, Gross unrealized losses	0.4	1.0
Greater than or equal to twelve months, Fair value	47.1	133.8
Greater than or equal to twelve months, Gross unrealized losses	36.4	55.6
Total, Fair value	106.0	206.0
Total, Gross unrealized losses	36.8	56.6
Other debt obligations
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	1,421.4	235.6
Less than twelve months, Gross unrealized losses	20.0	2.0
Greater than or equal to twelve months, Fair value	266.5	414.9
Greater than or equal to twelve months, Gross unrealized losses	31.0	49.2
Total, Fair value	1,687.9	650.5
Total, Gross unrealized losses	51.0	51.2
Equity securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	12.4	5.8
Less than twelve months, Gross unrealized losses	0.1	0.1
Greater than or equal to twelve months, Fair value	52.0	52.9
Greater than or equal to twelve months, Gross unrealized losses	9.2	8.4
Total, Fair value	64.4	58.7
Total, Gross unrealized losses	9.3	8.5
Principal Life Insurance Company | Fixed maturities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	8,777.9	1,382.1
Less than twelve months, Gross unrealized losses	292.4	16.2
Greater than or equal to twelve months, Fair value	2,148.3	3,037.3
Greater than or equal to twelve months, Gross unrealized losses	593.1	809.5
Total, Fair value	10,926.2	4,419.4
Total, Gross unrealized losses	885.5	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	1,141	224
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Twelve Months or Longer	379	488
Available-for-sale Securities in Unrealized Loss Position, Aggregate Losses On Investment Grade Investments, Less Than Twelve Months	275.6	13.3
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Percent Investment Grade (as a percent)	85.00%	71.00%
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Average Price (percent of carrying value to amortized cost)	93	84
Available-for-sale Securities in Unrealized Loss Positions, Percent Investment Grade, Less Than Twelve Months (as a percent)	92.00%	89.00%
Available-for-sale Securities in Unrealized Loss Positions, Average Price, Less Than Twelve Months (percent of carrying value to amortized cost)	97	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	211.4	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	$ 311.9	$ 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