Investments - Gross Unrealized Losses for Fixed Maturities (Details) $ in Millions	Sep. 30, 2025 USD ($) item	Dec. 31, 2024 USD ($) item
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	$ 5,521.2	$ 11,600.7
Fixed maturities, Less than twelve months, Gross unrealized losses	148.7	273.4
Fixed maturities, Greater than or equal to twelve months, Fair value	33,153.5	34,766.6
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	4,999.9	6,201.8
Fixed maturities, Total, Fair value	38,674.7	46,367.3
Fixed maturities, Total, Gross unrealized losses	5,148.6	6,475.2
U.S. government and agencies
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	527.8	909.1
Fixed maturities, Less than twelve months, Gross unrealized losses	8.2	17.4
Fixed maturities, Greater than or equal to twelve months, Fair value	656.5	810.6
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	247.5	283.1
Fixed maturities, Total, Fair value	1,184.3	1,719.7
Fixed maturities, Total, Gross unrealized losses	255.7	300.5
Non-U.S. governments
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	16.8	32.9
Fixed maturities, Less than twelve months, Gross unrealized losses	0.1	1.3
Fixed maturities, Greater than or equal to twelve months, Fair value	310.8	308.8
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	55.4	83.2
Fixed maturities, Total, Fair value	327.6	341.7
Fixed maturities, Total, Gross unrealized losses	55.5	84.5
States and political subdivisions
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	587.3	743.2
Fixed maturities, Less than twelve months, Gross unrealized losses	16.2	26.0
Fixed maturities, Greater than or equal to twelve months, Fair value	4,756.8	4,745.7
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	991.5	1,115.8
Fixed maturities, Total, Fair value	5,344.1	5,488.9
Fixed maturities, Total, Gross unrealized losses	1,007.7	1,141.8
Corporate debt securities
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	2,409.7	4,970.7
Fixed maturities, Less than twelve months, Gross unrealized losses	107.6	164.2
Fixed maturities, Greater than or equal to twelve months, Fair value	19,116.6	20,099.1
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	2,839.8	3,532.3
Fixed maturities, Total, Fair value	21,526.3	25,069.8
Fixed maturities, Total, Gross unrealized losses	2,947.4	3,696.5
Residential mortgage-backed pass-through securities
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	199.4	1,938.4
Fixed maturities, Less than twelve months, Gross unrealized losses	1.2	33.2
Fixed maturities, Greater than or equal to twelve months, Fair value	1,213.9	1,214.0
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	130.3	182.6
Fixed maturities, Total, Fair value	1,413.3	3,152.4
Fixed maturities, Total, Gross unrealized losses	131.5	215.8
Commercial mortgage-backed securities
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	354.1	691.7
Fixed maturities, Less than twelve months, Gross unrealized losses	3.2	8.2
Fixed maturities, Greater than or equal to twelve months, Fair value	3,577.6	3,860.1
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	321.6	442.6
Fixed maturities, Total, Fair value	3,931.7	4,551.8
Fixed maturities, Total, Gross unrealized losses	324.8	450.8
Collateralized debt obligations
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	640.8	582.2
Fixed maturities, Less than twelve months, Gross unrealized losses	2.9	1.3
Fixed maturities, Greater than or equal to twelve months, Fair value	21.4	29.8
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	4.4	5.0
Fixed maturities, Total, Fair value	662.2	612.0
Fixed maturities, Total, Gross unrealized losses	7.3	6.3
Other debt obligations
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	785.3	1,732.5
Fixed maturities, Less than twelve months, Gross unrealized losses	9.3	21.8
Fixed maturities, Greater than or equal to twelve months, Fair value	3,499.9	3,698.5
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	409.4	557.2
Fixed maturities, Total, Fair value	4,285.2	5,431.0
Fixed maturities, Total, Gross unrealized losses	418.7	579.0
Principal Life Insurance Company
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	5,210.8	10,805.4
Fixed maturities, Less than twelve months, Gross unrealized losses	138.2	252.3
Fixed maturities, Greater than or equal to twelve months, Fair value	32,576.7	34,145.3
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	4,936.9	6,121.5
Fixed maturities, Total, Fair value	37,787.5	44,950.7
Fixed maturities, Total, Gross unrealized losses	$ 5,075.1	$ 6,373.8
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%	97.00%
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Average Price (percent of carrying value to amortized cost)	88.00%	88.00%
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less Than Twelve Months \| item	878	1,747
Available-for-sale Securities in Unrealized Loss Positions, Average Price, Less Than Twelve Months (percent of carrying value to amortized cost)	97.00%	98.00%
Available-for-sale Securities in Unrealized Loss Positions, Percent Investment Grade, Less Than Twelve Months (as a percent)	95.00%	96.00%
Available-for-sale Securities in Unrealized Loss Position, Aggregate Losses On Investment Grade Investments, Less Than Twelve Months	$ 134.5	$ 245.4
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Twelve Months or Longer \| item	5,783	6,219
Available-for-sale Securities in Unrealized Loss Positions, Average Price, Twelve Months or Longer (percent of carrying value to amortized cost)	87.00%	85.00%
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	$ 984.6	$ 1,108.0
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%	81.00%
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	$ 2,790.4	$ 3,470.0
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)	87.00%	85.00%
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	$ 320.9	$ 441.5
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)	92.00%	90.00%
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	$ 374.3	$ 490.8
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)	86.00%	83.00%

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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