Investments (Details 6) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Fixed maturities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	$ 7,130.4	$ 9,313.2
Less than twelve months, Gross unrealized losses	182.7	356.4
Greater than or equal to twelve months, Fair value	2,166.5	2,256.0
Greater than or equal to twelve months, Gross unrealized losses	351.9	450.2
Total, Fair value	9,296.9	11,569.2
Total, Gross unrealized losses	534.6	806.6
U.S. government and agencies
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	464.6	526.8
Less than twelve months, Gross unrealized losses	30.5	49.6
Greater than or equal to twelve months, Fair value	15.8	9.2
Greater than or equal to twelve months, Gross unrealized losses	1.1	0.8
Total, Fair value	480.4	536.0
Total, Gross unrealized losses	31.6	50.4
Non-U.S. governments
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	75.4	78.1
Less than twelve months, Gross unrealized losses	4.8	5.1
Greater than or equal to twelve months, Fair value	5.9	5.8
Greater than or equal to twelve months, Gross unrealized losses	0.3	0.1
Total, Fair value	81.3	83.9
Total, Gross unrealized losses	5.1	5.2
States and political subdivisions
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	915.4	1,338.6
Less than twelve months, Gross unrealized losses	32.5	75.3
Greater than or equal to twelve months, Fair value	79.2	46.1
Greater than or equal to twelve months, Gross unrealized losses	9.3	10.4
Total, Fair value	994.6	1,384.7
Total, Gross unrealized losses	41.8	85.7
Corporate debt securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	3,213.9	4,087.9
Less than twelve months, Gross unrealized losses	71.4	155.4
Greater than or equal to twelve months, Fair value	1,123.3	1,278.1
Greater than or equal to twelve months, Gross unrealized losses	138.5	165.0
Total, Fair value	4,337.2	5,366.0
Total, Gross unrealized losses	209.9	320.4
Residential mortgage-backed pass-through securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	936.5	1,150.3
Less than twelve months, Gross unrealized losses	24.4	38.2
Greater than or equal to twelve months, Fair value	112.7	85.9
Greater than or equal to twelve months, Gross unrealized losses	7.9	8.5
Total, Fair value	1,049.2	1,236.2
Total, Gross unrealized losses	32.3	46.7
Commercial mortgage-backed securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	545.1	683.7
Less than twelve months, Gross unrealized losses	9.1	15.3
Greater than or equal to twelve months, Fair value	472.8	495.6
Greater than or equal to twelve months, Gross unrealized losses	140.8	206.9
Total, Fair value	1,017.9	1,179.3
Total, Gross unrealized losses	149.9	222.2
Collateralized debt obligations
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	113.3	88.8
Less than twelve months, Gross unrealized losses	1.5	1.4
Greater than or equal to twelve months, Fair value	52.5	47.4
Greater than or equal to twelve months, Gross unrealized losses	30.2	33.1
Total, Fair value	165.8	136.2
Total, Gross unrealized losses	31.7	34.5
Other debt obligations
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	866.2	1,359.0
Less than twelve months, Gross unrealized losses	8.5	16.1
Greater than or equal to twelve months, Fair value	304.3	287.9
Greater than or equal to twelve months, Gross unrealized losses	23.8	25.4
Total, Fair value	1,170.5	1,646.9
Total, Gross unrealized losses	32.3	41.5
Equity securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value		16.7
Less than twelve months, Gross unrealized losses		0.3
Greater than or equal to twelve months, Fair value	51.0	48.3
Greater than or equal to twelve months, Gross unrealized losses	10.2	13.0
Total, Fair value	51.0	65.0
Total, Gross unrealized losses	10.2	13.3
Principal Life Insurance Company | Fixed maturities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	7,008.9	8,899.5
Less than twelve months, Gross unrealized losses	173.2	339.8
Greater than or equal to twelve months, Fair value	1,929.8	2,005.9
Greater than or equal to twelve months, Gross unrealized losses	324.0	412.7
Total, Fair value	8,938.7	10,905.4
Total, Gross unrealized losses	497.2	752.5
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	862	1,154
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Twelve Months or Longer	347	359
Available-for-sale Securities in Unrealized Loss Position, Aggregate Losses On Investment Grade Investments, Less Than Twelve Months	166.6	325.9
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Percent Investment Grade (as a percent)	87.00%	87.00%
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Average Price (percent of carrying value to amortized cost)	95	94
Available-for-sale Securities in Unrealized Loss Positions, Percent Investment Grade, Less Than Twelve Months (as a percent)	95.00%	94.00%
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)	86	83
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	110.9	127.6
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)	89	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	$ 140.8	$ 206.9
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)	77	71