Investments - Gross Unrealized Losses for Fixed Maturities (Details) $ in Millions	Dec. 31, 2024 USD ($) security	Dec. 31, 2023 USD ($) security
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	$ 11,600.7	$ 6,667.6
Fixed maturities, Less than twelve months, Gross unrealized losses	273.4	180.0
Fixed maturities, Greater than or equal to twelve months, Fair value	34,766.6	41,511.0
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	6,201.8	5,692.9
Fixed maturities, Total, Fair value	46,367.3	48,178.6
Fixed maturities, Total, Gross unrealized losses	6,475.2	5,872.9
U.S. government and agencies
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	909.1	316.4
Fixed maturities, Less than twelve months, Gross unrealized losses	17.4	3.1
Fixed maturities, Greater than or equal to twelve months, Fair value	810.6	997.1
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	283.1	232.6
Fixed maturities, Total, Fair value	1,719.7	1,313.5
Fixed maturities, Total, Gross unrealized losses	300.5	235.7
Non-U.S. governments
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	32.9	41.9
Fixed maturities, Less than twelve months, Gross unrealized losses	1.3	1.7
Fixed maturities, Greater than or equal to twelve months, Fair value	308.8	331.3
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	83.2	57.9
Fixed maturities, Total, Fair value	341.7	373.2
Fixed maturities, Total, Gross unrealized losses	84.5	59.6
States and political subdivisions
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	743.2	596.6
Fixed maturities, Less than twelve months, Gross unrealized losses	26.0	19.0
Fixed maturities, Greater than or equal to twelve months, Fair value	4,745.7	5,075.1
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	1,115.8	914.8
Fixed maturities, Total, Fair value	5,488.9	5,671.7
Fixed maturities, Total, Gross unrealized losses	1,141.8	933.8
Corporate debt securities
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	4,970.7	3,210.1
Fixed maturities, Less than twelve months, Gross unrealized losses	164.2	126.9
Fixed maturities, Greater than or equal to twelve months, Fair value	20,099.1	22,929.1
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	3,532.3	3,064.4
Fixed maturities, Total, Fair value	25,069.8	26,139.2
Fixed maturities, Total, Gross unrealized losses	3,696.5	3,191.3
Residential mortgage-backed pass-through securities
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	1,938.4	633.0
Fixed maturities, Less than twelve months, Gross unrealized losses	33.2	4.9
Fixed maturities, Greater than or equal to twelve months, Fair value	1,214.0	1,275.6
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	182.6	159.7
Fixed maturities, Total, Fair value	3,152.4	1,908.6
Fixed maturities, Total, Gross unrealized losses	215.8	164.6
Commercial mortgage-backed securities
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	691.7	280.3
Fixed maturities, Less than twelve months, Gross unrealized losses	8.2	4.1
Fixed maturities, Greater than or equal to twelve months, Fair value	3,860.1	4,192.4
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	442.6	650.1
Fixed maturities, Total, Fair value	4,551.8	4,472.7
Fixed maturities, Total, Gross unrealized losses	450.8	654.2
Collateralized debt obligations
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	582.2	405.6
Fixed maturities, Less than twelve months, Gross unrealized losses	1.3	1.8
Fixed maturities, Greater than or equal to twelve months, Fair value	29.8	2,301.6
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	5.0	25.4
Fixed maturities, Total, Fair value	612.0	2,707.2
Fixed maturities, Total, Gross unrealized losses	6.3	27.2
Other debt obligations
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	1,732.5	1,183.7
Fixed maturities, Less than twelve months, Gross unrealized losses	21.8	18.5
Fixed maturities, Greater than or equal to twelve months, Fair value	3,698.5	4,408.8
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	557.2	588.0
Fixed maturities, Total, Fair value	5,431.0	5,592.5
Fixed maturities, Total, Gross unrealized losses	579.0	606.5
Principal Life Insurance Company
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	10,805.4	5,836.0
Fixed maturities, Less than twelve months, Gross unrealized losses	252.3	140.5
Fixed maturities, Greater than or equal to twelve months, Fair value	34,145.3	40,717.9
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	6,121.5	5,576.6
Fixed maturities, Total, Fair value	44,950.7	46,553.9
Fixed maturities, Total, Gross unrealized losses	$ 6,373.8	$ 5,717.1
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)	88.00%	89.00%
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less Than Twelve Months \| security	1,747	983
Available-for-sale Securities in Unrealized Loss Positions, Average Price, Less Than Twelve Months (percent of carrying value to amortized cost)	98.00%	98.00%
Available-for-sale Securities in Unrealized Loss Positions, Percent Investment Grade, Less Than Twelve Months (as a percent)	96.00%	91.00%
Available-for-sale Securities in Unrealized Loss Position, Aggregate Losses On Investment Grade Investments, Less Than Twelve Months	$ 245.4	$ 131.2
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Twelve Months or Longer \| security	6,219	7,042
Available-for-sale Securities in Unrealized Loss Positions, Average Price, Twelve Months or Longer (percent of carrying value to amortized cost)	85.00%	88.00%
Principal Life Insurance Company \| States and political subdivisions
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	$ 1,108.0	$ 906.9
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)	81.00%	85.00%
Principal Life Insurance Company \| Corporate debt securities
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	$ 3,470.0	$ 2,968.9
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)	85.00%	88.00%
Principal Life Insurance Company \| Commercial mortgage-backed securities
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	$ 441.5	$ 645.6
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)	90.00%	87.00%
Principal Life Insurance Company \| Collateralized mortgage obligation security
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	$ 490.8	$ 472.9
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)	83.00%	85.00%

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