Investments - Gross Unrealized Losses for Fixed Maturities and Equity Securities (Details) $ in Millions	Jun. 30, 2016 USD ($) item	Dec. 31, 2015 USD ($) item
Fixed maturities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	$ 3,244.1	$ 14,421.5
Less than twelve months, Gross unrealized losses	61.1	395.9
Greater than or equal to twelve months, Fair value	3,105.2	2,453.0
Greater than or equal to twelve months, Gross unrealized losses	378.9	427.3
Total, Fair value	6,349.3	16,874.5
Total, Gross unrealized losses	440.0	823.2
U.S. government and agencies
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	8.8	590.4
Less than twelve months, Gross unrealized losses		7.6
Greater than or equal to twelve months, Fair value	3.6	40.5
Greater than or equal to twelve months, Gross unrealized losses		0.7
Total, Fair value	12.4	630.9
Total, Gross unrealized losses		8.3
Non-U.S. governments
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	1.9	86.3
Less than twelve months, Gross unrealized losses		3.1
Greater than or equal to twelve months, Fair value	10.7	16.1
Greater than or equal to twelve months, Gross unrealized losses	1.0	1.9
Total, Fair value	12.6	102.4
Total, Gross unrealized losses	1.0	5.0
States and political subdivisions
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	2.2	692.0
Less than twelve months, Gross unrealized losses		19.0
Greater than or equal to twelve months, Fair value	17.2	6.5
Greater than or equal to twelve months, Gross unrealized losses	0.4	0.4
Total, Fair value	19.4	698.5
Total, Gross unrealized losses	0.4	19.4
Corporate debt securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	1,486.0	7,975.7
Less than twelve months, Gross unrealized losses	40.7	309.3
Greater than or equal to twelve months, Fair value	1,743.7	1,375.0
Greater than or equal to twelve months, Gross unrealized losses	279.7	328.9
Total, Fair value	3,229.7	9,350.7
Total, Gross unrealized losses	320.4	638.2
Residential mortgage-backed pass-through securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	0.4	656.7
Less than twelve months, Gross unrealized losses		6.7
Greater than or equal to twelve months, Fair value	44.0	147.9
Greater than or equal to twelve months, Gross unrealized losses	0.3	5.2
Total, Fair value	44.4	804.6
Total, Gross unrealized losses	0.3	11.9
Commercial mortgage-backed securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	790.5	1,480.8
Less than twelve months, Gross unrealized losses	13.0	27.3
Greater than or equal to twelve months, Fair value	517.6	299.5
Greater than or equal to twelve months, Gross unrealized losses	52.3	50.7
Total, Fair value	1,308.1	1,780.3
Total, Gross unrealized losses	65.3	78.0
Collateralized debt obligations
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	358.5	426.9
Less than twelve months, Gross unrealized losses	1.7	3.8
Greater than or equal to twelve months, Fair value	293.3	164.0
Greater than or equal to twelve months, Gross unrealized losses	26.2	22.8
Total, Fair value	651.8	590.9
Total, Gross unrealized losses	27.9	26.6
Other debt obligations
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	595.8	2,512.7
Less than twelve months, Gross unrealized losses	5.7	19.1
Greater than or equal to twelve months, Fair value	475.1	403.5
Greater than or equal to twelve months, Gross unrealized losses	19.0	16.7
Total, Fair value	1,070.9	2,916.2
Total, Gross unrealized losses	24.7	35.8
Equity securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	0.4	0.8
Less than twelve months, Gross unrealized losses	1.5	1.0
Greater than or equal to twelve months, Fair value	33.5	32.7
Greater than or equal to twelve months, Gross unrealized losses	12.3	13.2
Total, Fair value	33.9	33.5
Total, Gross unrealized losses	13.8	14.2
Principal Life Insurance Company | Fixed maturities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	3,107.1	13,673.9
Less than twelve months, Gross unrealized losses	53.5	376.3
Greater than or equal to twelve months, Fair value	2,948.2	2,306.1
Greater than or equal to twelve months, Gross unrealized losses	357.2	400.7
Total, Fair value	6,055.3	15,980.0
Total, Gross unrealized losses	$ 410.7	$ 777.0
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 | item	387	1,725
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Twelve Months or Longer | item	506	404
Available-for-sale Securities in Unrealized Loss Position, Aggregate Losses On Investment Grade Investments, Less Than Twelve Months	$ 35.1	$ 298.1
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Percent Investment Grade (as a percent)	72.00%	87.00%
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Average Price (percent of carrying value to amortized cost) | item	94	95
Available-for-sale Securities in Unrealized Loss Positions, Percent Investment Grade, Less Than Twelve Months (as a percent)	75.00%	90.00%
Available-for-sale Securities in Unrealized Loss Positions, Average Price, Less Than Twelve Months (percent of carrying value to amortized cost) | item	98	97
Available-for-sale Securities in Unrealized Loss Positions, Average Price, Twelve Months or Longer (percent of carrying value to amortized cost) | item	89	85
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	$ 259.0	$ 304.2
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) | item	86	80
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	$ 52.4	$ 50.7
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) | item	91	86