Investments - Gross Unrealized Losses for Fixed Maturities and Equity Securities (Details) $ in Millions	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($) item
Fixed maturities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	$ 9,822.3	$ 14,955.3
Less than twelve months, Gross unrealized losses	92.3	396.6
Greater than or equal to twelve months, Fair value	4,834.9	2,511.1
Greater than or equal to twelve months, Gross unrealized losses	174.7	252.2
Total, Fair value	14,657.2	17,466.4
Total, Gross unrealized losses	267.0	648.8
U.S. government and agencies
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	294.2	570.3
Less than twelve months, Gross unrealized losses	2.2	10.9
Greater than or equal to twelve months, Fair value	180.9	8.2
Greater than or equal to twelve months, Gross unrealized losses	5.5
Total, Fair value	475.1	578.5
Total, Gross unrealized losses	7.7	10.9
Non-U.S. governments
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	111.0	198.0
Less than twelve months, Gross unrealized losses	1.7	5.4
Greater than or equal to twelve months, Fair value	22.1	12.2
Greater than or equal to twelve months, Gross unrealized losses	1.9	2.0
Total, Fair value	133.1	210.2
Total, Gross unrealized losses	3.6	7.4
States and political subdivisions
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	720.0	2,229.4
Less than twelve months, Gross unrealized losses	5.0	86.6
Greater than or equal to twelve months, Fair value	437.7	4.8
Greater than or equal to twelve months, Gross unrealized losses	10.9	0.5
Total, Fair value	1,157.7	2,234.2
Total, Gross unrealized losses	15.9	87.1
Corporate debt securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	3,871.5	6,559.7
Less than twelve months, Gross unrealized losses	43.4	189.2
Greater than or equal to twelve months, Fair value	1,644.3	1,285.6
Greater than or equal to twelve months, Gross unrealized losses	60.7	161.6
Total, Fair value	5,515.8	7,845.3
Total, Gross unrealized losses	104.1	350.8
Residential mortgage-backed pass-through securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	354.4	1,265.6
Less than twelve months, Gross unrealized losses	2.0	29.8
Greater than or equal to twelve months, Fair value	734.5	16.0
Greater than or equal to twelve months, Gross unrealized losses	19.3	0.8
Total, Fair value	1,088.9	1,281.6
Total, Gross unrealized losses	21.3	30.6
Commercial mortgage-backed securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	1,342.7	1,637.2
Less than twelve months, Gross unrealized losses	19.9	41.0
Greater than or equal to twelve months, Fair value	820.3	612.5
Greater than or equal to twelve months, Gross unrealized losses	38.5	46.9
Total, Fair value	2,163.0	2,249.7
Total, Gross unrealized losses	58.4	87.9
Collateralized debt obligations
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	460.9	265.7
Less than twelve months, Gross unrealized losses	2.1	0.9
Greater than or equal to twelve months, Fair value	38.3	195.6
Greater than or equal to twelve months, Gross unrealized losses	13.4	23.4
Total, Fair value	499.2	461.3
Total, Gross unrealized losses	15.5	24.3
Other debt obligations
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	2,667.6	2,229.4
Less than twelve months, Gross unrealized losses	16.0	32.8
Greater than or equal to twelve months, Fair value	956.8	376.2
Greater than or equal to twelve months, Gross unrealized losses	24.5	17.0
Total, Fair value	3,624.4	2,605.6
Total, Gross unrealized losses	40.5	49.8
Equity portfolios
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value		18.2
Less than twelve months, Gross unrealized losses		0.4
Greater than or equal to twelve months, Fair value	40.4	35.4
Greater than or equal to twelve months, Gross unrealized losses	5.4	10.5
Total, Fair value	40.4	53.6
Total, Gross unrealized losses	5.4	10.9
Principal Life Insurance Company | Fixed maturities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	9,360.9	14,549.4
Less than twelve months, Gross unrealized losses	75.4	384.6
Greater than or equal to twelve months, Fair value	4,698.6	2,369.5
Greater than or equal to twelve months, Gross unrealized losses	164.3	230.5
Total, Fair value	14,059.5	16,918.9
Total, Gross unrealized losses	$ 239.7	$ 615.1
Available-for-sale Securities, Continuous Unrealized Loss Position, Qualitative Disclosure
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Percent Investment Grade (as a percent)	97.00%	94.00%
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Average Price (percent of carrying value to amortized cost) | item	98	96
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less Than Twelve Months | item	1,209	1,911
Available-for-sale Securities in Unrealized Loss Positions, Average Price, Less Than Twelve Months (percent of carrying value to amortized cost) | item	99	97
Available-for-sale Securities in Unrealized Loss Positions, Percent Investment Grade, Less Than Twelve Months (as a percent)	98.00%	98.00%
Available-for-sale Securities in Unrealized Loss Position, Aggregate Losses On Investment Grade Investments, Less Than Twelve Months	$ 71.5	$ 374.1
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Twelve Months or Longer | item	775	453
Available-for-sale Securities in Unrealized Loss Positions, Average Price, Twelve Months or Longer (percent of carrying value to amortized cost) | item	97	91
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	$ 52.2	$ 141.9
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) | item	97	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	$ 38.1	$ 46.9
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) | item	96	93