Investments - Gross Unrealized Losses for Fixed Maturities (Details) $ in Millions	Mar. 31, 2022 USD ($) item	Dec. 31, 2021 USD ($) item
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	$ 36,086.0	$ 14,561.3
Fixed maturities, Less than twelve months, Gross unrealized losses	1,827.8	227.1
Fixed maturities, Greater than or equal to twelve months, Fair value	3,393.8	3,042.6
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	429.8	179.3
Fixed maturities, Total, Fair value	39,479.8	17,603.9
Fixed maturities, Total, Gross unrealized losses	2,257.6	406.4
U.S. government and agencies
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	1,235.6	129.3
Fixed maturities, Less than twelve months, Gross unrealized losses	20.1	3.4
Fixed maturities, Greater than or equal to twelve months, Fair value	254.3	482.9
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	56.6	34.0
Fixed maturities, Total, Fair value	1,489.9	612.2
Fixed maturities, Total, Gross unrealized losses	76.7	37.4
Non-U.S. governments
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	232.9	57.8
Fixed maturities, Less than twelve months, Gross unrealized losses	12.9	2.0
Fixed maturities, Greater than or equal to twelve months, Fair value	5.6
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	1.0
Fixed maturities, Total, Fair value	238.5	57.8
Fixed maturities, Total, Gross unrealized losses	13.9	2.0
States and political subdivisions
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	3,091.5	690.2
Fixed maturities, Less than twelve months, Gross unrealized losses	245.7	10.5
Fixed maturities, Greater than or equal to twelve months, Fair value	154.1	102.3
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	23.7	6.1
Fixed maturities, Total, Fair value	3,245.6	792.5
Fixed maturities, Total, Gross unrealized losses	269.4	16.6
Corporate debt securities
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	18,061.6	5,281.6
Fixed maturities, Less than twelve months, Gross unrealized losses	1,024.1	121.2
Fixed maturities, Greater than or equal to twelve months, Fair value	1,504.8	1,327.5
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	252.9	101.5
Fixed maturities, Total, Fair value	19,566.4	6,609.1
Fixed maturities, Total, Gross unrealized losses	1,277.0	222.7
Residential mortgage-backed pass-through securities
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	1,455.1	1,562.6
Fixed maturities, Less than twelve months, Gross unrealized losses	76.5	22.2
Fixed maturities, Greater than or equal to twelve months, Fair value	104.4	194.9
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	9.3	6.3
Fixed maturities, Total, Fair value	1,559.5	1,757.5
Fixed maturities, Total, Gross unrealized losses	85.8	28.5
Commercial mortgage-backed securities
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	4,337.2	1,297.4
Fixed maturities, Less than twelve months, Gross unrealized losses	186.3	15.6
Fixed maturities, Greater than or equal to twelve months, Fair value	331.7	299.6
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	30.0	15.7
Fixed maturities, Total, Fair value	4,668.9	1,597.0
Fixed maturities, Total, Gross unrealized losses	216.3	31.3
Collateralized debt obligations
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	2,786.5	1,592.5
Fixed maturities, Less than twelve months, Gross unrealized losses	23.7	2.8
Fixed maturities, Greater than or equal to twelve months, Fair value	592.1	424.4
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	10.0	6.7
Fixed maturities, Total, Fair value	3,378.6	2,016.9
Fixed maturities, Total, Gross unrealized losses	33.7	9.5
Other debt obligations
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	4,885.6	3,949.9
Fixed maturities, Less than twelve months, Gross unrealized losses	238.5	49.4
Fixed maturities, Greater than or equal to twelve months, Fair value	446.8	211.0
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	46.3	9.0
Fixed maturities, Total, Fair value	5,332.4	4,160.9
Fixed maturities, Total, Gross unrealized losses	284.8	58.4
Principal Life Insurance Company
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	34,890.6	13,052.3
Fixed maturities, Less than twelve months, Gross unrealized losses	1,753.3	150.9
Fixed maturities, Greater than or equal to twelve months, Fair value	3,248.9	2,740.3
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	391.5	137.1
Fixed maturities, Total, Fair value	38,139.5	15,792.6
Fixed maturities, Total, Gross unrealized losses	$ 2,144.8	$ 288.0
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)	94.00%	91.00%
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Average Price (percent of carrying value to amortized cost)	95.00%	98.00%
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less Than Twelve Months | item	5,353	1,805
Available-for-sale Securities in Unrealized Loss Positions, Average Price, Less Than Twelve Months (percent of carrying value to amortized cost)	95.00%	99.00%
Available-for-sale Securities in Unrealized Loss Positions, Percent Investment Grade, Less Than Twelve Months (as a percent)	94.00%	90.00%
Available-for-sale Securities in Unrealized Loss Position, Aggregate Losses On Investment Grade Investments, Less Than Twelve Months	$ 1,635.1	$ 138.9
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Twelve Months or Longer | item	572	459
Available-for-sale Securities in Unrealized Loss Positions, Average Price, Twelve Months or Longer (percent of carrying value to amortized cost)	89.00%	95.00%
Principal Life Insurance Company | U.S. government and agencies
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	$ 56.6	$ 30.9
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)	82.00%	93.00%
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	$ 215.7	$ 67.1
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)	87.00%	95.00%
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	$ 29.3	$ 15.3
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%	95.00%
Principal Life Insurance Company | Collateralized debt obligations
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses		$ 6.7
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)		98.00%
Principal Life Insurance Company | Other debt obligations
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	$ 29.4
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)	91.00%