Investments - Gross Unrealized Losses for Fixed Maturities and Equity Securities (Details) $ in Millions	Jun. 30, 2018 USD ($) item	Dec. 31, 2017 USD ($) item
Fixed maturities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	$ 28,492.8	$ 9,822.3
Less than twelve months, Gross unrealized losses	726.7	92.3
Greater than or equal to twelve months, Fair value	4,453.9	4,834.9
Greater than or equal to twelve months, Gross unrealized losses	277.9	174.7
Total, Fair value	32,946.7	14,657.2
Total, Gross unrealized losses	1,004.6	267.0
U.S. government and agencies
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	574.4	294.2
Less than twelve months, Gross unrealized losses	11.4	2.2
Greater than or equal to twelve months, Fair value	175.4	180.9
Greater than or equal to twelve months, Gross unrealized losses	10.0	5.5
Total, Fair value	749.8	475.1
Total, Gross unrealized losses	21.4	7.7
Non-U.S. governments
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	376.7	111.0
Less than twelve months, Gross unrealized losses	14.7	1.7
Greater than or equal to twelve months, Fair value	27.7	22.1
Greater than or equal to twelve months, Gross unrealized losses	2.1	1.9
Total, Fair value	404.4	133.1
Total, Gross unrealized losses	16.8	3.6
States and political subdivisions
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	2,071.6	720.0
Less than twelve months, Gross unrealized losses	43.1	5.0
Greater than or equal to twelve months, Fair value	422.7	437.7
Greater than or equal to twelve months, Gross unrealized losses	23.9	10.9
Total, Fair value	2,494.3	1,157.7
Total, Gross unrealized losses	67.0	15.9
Corporate debt securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	16,927.5	3,871.5
Less than twelve months, Gross unrealized losses	517.2	43.4
Greater than or equal to twelve months, Fair value	1,388.1	1,644.3
Greater than or equal to twelve months, Gross unrealized losses	105.6	60.7
Total, Fair value	18,315.6	5,515.8
Total, Gross unrealized losses	622.8	104.1
Residential mortgage-backed pass-through securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	1,096.2	354.4
Less than twelve months, Gross unrealized losses	22.1	2.0
Greater than or equal to twelve months, Fair value	676.7	734.5
Greater than or equal to twelve months, Gross unrealized losses	38.5	19.3
Total, Fair value	1,772.9	1,088.9
Total, Gross unrealized losses	60.6	21.3
Commercial mortgage-backed securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	2,215.6	1,342.7
Less than twelve months, Gross unrealized losses	45.8	19.9
Greater than or equal to twelve months, Fair value	820.3	820.3
Greater than or equal to twelve months, Gross unrealized losses	51.7	38.5
Total, Fair value	3,035.9	2,163.0
Total, Gross unrealized losses	97.5	58.4
Collateralized debt obligations
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	1,115.1	460.9
Less than twelve months, Gross unrealized losses	2.5	2.1
Greater than or equal to twelve months, Fair value	50.0	38.3
Greater than or equal to twelve months, Gross unrealized losses	12.3	13.4
Total, Fair value	1,165.1	499.2
Total, Gross unrealized losses	14.8	15.5
Other debt obligations
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	4,115.7	2,667.6
Less than twelve months, Gross unrealized losses	69.9	16.0
Greater than or equal to twelve months, Fair value	893.0	956.8
Greater than or equal to twelve months, Gross unrealized losses	33.8	24.5
Total, Fair value	5,008.7	3,624.4
Total, Gross unrealized losses	103.7	40.5
Equity securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Greater than or equal to twelve months, Fair value		40.4
Greater than or equal to twelve months, Gross unrealized losses		5.4
Total, Fair value		40.4
Total, Gross unrealized losses		5.4
Principal Life Insurance Company | Fixed maturities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	27,795.1	9,360.9
Less than twelve months, Gross unrealized losses	702.0	75.4
Greater than or equal to twelve months, Fair value	4,262.2	4,698.6
Greater than or equal to twelve months, Gross unrealized losses	264.9	164.3
Total, Fair value	32,057.3	14,059.5
Total, Gross unrealized losses	$ 966.9	$ 239.7
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)	96.00%	97.00%
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Average Price (percent of carrying value to amortized cost) | item	97	98
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less Than Twelve Months | item	3,384	1,209
Available-for-sale Securities in Unrealized Loss Positions, Average Price, Less Than Twelve Months (percent of carrying value to amortized cost) | item	98	99
Available-for-sale Securities in Unrealized Loss Positions, Percent Investment Grade, Less Than Twelve Months (as a percent)	96.00%	98.00%
Available-for-sale Securities in Unrealized Loss Position, Aggregate Losses On Investment Grade Investments, Less Than Twelve Months	$ 671.6	$ 71.5
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Twelve Months or Longer | item	761	775
Available-for-sale Securities in Unrealized Loss Positions, Average Price, Twelve Months or Longer (percent of carrying value to amortized cost) | item	94	97
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	$ 94.4	$ 52.2
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	93	97
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	$ 50.9	$ 38.1
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) | item	94	96