Investments (Details 6) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Fixed maturities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	$ 4,459.9	$ 9,313.2
Less than twelve months, Gross unrealized losses	36.6	356.4
Greater than or equal to twelve months, Fair value	3,586.7	2,256.0
Greater than or equal to twelve months, Gross unrealized losses	284.4	450.2
Total, Fair value	8,046.6	11,569.2
Total, Gross unrealized losses	321.0	806.6
U.S. government and agencies
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	258.7	526.8
Less than twelve months, Gross unrealized losses	1.2	49.6
Greater than or equal to twelve months, Fair value	210.5	9.2
Greater than or equal to twelve months, Gross unrealized losses	7.2	0.8
Total, Fair value	469.2	536.0
Total, Gross unrealized losses	8.4	50.4
Non-U.S. governments
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	36.0	78.1
Less than twelve months, Gross unrealized losses	0.6	5.1
Greater than or equal to twelve months, Fair value	11.6	5.8
Greater than or equal to twelve months, Gross unrealized losses	0.3	0.1
Total, Fair value	47.6	83.9
Total, Gross unrealized losses	0.9	5.2
States and political subdivisions
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	169.8	1,338.6
Less than twelve months, Gross unrealized losses	1.3	75.3
Greater than or equal to twelve months, Fair value	359.5	46.1
Greater than or equal to twelve months, Gross unrealized losses	14.4	10.4
Total, Fair value	529.3	1,384.7
Total, Gross unrealized losses	15.7	85.7
Corporate debt securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	2,489.2	4,087.9
Less than twelve months, Gross unrealized losses	24.9	155.4
Greater than or equal to twelve months, Fair value	1,459.5	1,278.1
Greater than or equal to twelve months, Gross unrealized losses	131.6	165.0
Total, Fair value	3,948.7	5,366.0
Total, Gross unrealized losses	156.5	320.4
Residential mortgage-backed pass-through securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	175.7	1,150.3
Less than twelve months, Gross unrealized losses	0.7	38.2
Greater than or equal to twelve months, Fair value	504.1	85.9
Greater than or equal to twelve months, Gross unrealized losses	14.6	8.5
Total, Fair value	679.8	1,236.2
Total, Gross unrealized losses	15.3	46.7
Commercial mortgage-backed securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	315.1	683.7
Less than twelve months, Gross unrealized losses	3.0	15.3
Greater than or equal to twelve months, Fair value	472.4	495.6
Greater than or equal to twelve months, Gross unrealized losses	75.2	206.9
Total, Fair value	787.5	1,179.3
Total, Gross unrealized losses	78.2	222.2
Collateralized debt obligations
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	147.5	88.8
Less than twelve months, Gross unrealized losses	1.6	1.4
Greater than or equal to twelve months, Fair value	82.9	47.4
Greater than or equal to twelve months, Gross unrealized losses	18.1	33.1
Total, Fair value	230.4	136.2
Total, Gross unrealized losses	19.7	34.5
Other debt obligations
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	867.9	1,359.0
Less than twelve months, Gross unrealized losses	3.3	16.1
Greater than or equal to twelve months, Fair value	486.2	287.9
Greater than or equal to twelve months, Gross unrealized losses	23.0	25.4
Total, Fair value	1,354.1	1,646.9
Total, Gross unrealized losses	26.3	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	50.9	48.3
Greater than or equal to twelve months, Gross unrealized losses	10.4	13.0
Total, Fair value	50.9	65.0
Total, Gross unrealized losses	10.4	13.3
Principal Life Insurance Company Only | Fixed maturities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	4,426.0	8,899.5
Less than twelve months, Gross unrealized losses	36.4	339.8
Greater than or equal to twelve months, Fair value	3,407.3	2,005.9
Greater than or equal to twelve months, Gross unrealized losses	260.3	412.7
Total, Fair value	7,833.3	10,905.4
Total, Gross unrealized losses	296.7	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	591	1,154
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Twelve Months or Longer	507	359
Available-for-sale Securities in Unrealized Loss Position, Aggregate Losses On Investment Grade Investments, Less Than Twelve Months	24.4	325.9
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Percent Investment Grade (as a percent)	83.00%	87.00%
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Average Price (percent of carrying value to amortized cost)	96	94
Available-for-sale Securities in Unrealized Loss Positions, Percent Investment Grade, Less Than Twelve Months (as a percent)	79.00%	94.00%
Available-for-sale Securities in Unrealized Loss Positions, Average Price, Less Than Twelve Months (percent of carrying value to amortized cost)	99	96
Available-for-sale Securities in Unrealized Loss Positions, Average Price, Twelve Months or Longer (percent of carrying value to amortized cost)	93	83
Principal Life Insurance Company Only | Corporate debt securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Greater than or equal to twelve months, Gross unrealized losses	108.0	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)	92	89
Principal Life Insurance Company Only | Commercial mortgage-backed securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Greater than or equal to twelve months, Gross unrealized losses	$ 75.2	$ 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)	86	71