Investments - Gross Unrealized Losses for Fixed Maturities (Details) $ in Millions	Sep. 30, 2021 USD ($) item	Dec. 31, 2020 USD ($) item
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	$ 10,647.8	$ 5,909.4
Fixed maturities, Less than twelve months, Gross unrealized losses	208.9	138.8
Fixed maturities, Greater than or equal to twelve months, Fair value	1,826.6	1,399.1
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	119.1	67.4
Fixed maturities, Total, Fair value	12,474.4	7,308.5
Fixed maturities, Total, Gross unrealized losses	328.0	206.2
U.S. government and agencies
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	400.1	351.1
Fixed maturities, Less than twelve months, Gross unrealized losses	14.9	10.4
Fixed maturities, Greater than or equal to twelve months, Fair value	150.8
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	25.7
Fixed maturities, Total, Fair value	550.9	351.1
Fixed maturities, Total, Gross unrealized losses	40.6	10.4
Non-U.S. governments
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	47.7
Fixed maturities, Less than twelve months, Gross unrealized losses	0.8
Fixed maturities, Total, Fair value	47.7
Fixed maturities, Total, Gross unrealized losses	0.8
States and political subdivisions
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	632.3	363.5
Fixed maturities, Less than twelve months, Gross unrealized losses	10.9	12.5
Fixed maturities, Greater than or equal to twelve months, Fair value	245.1
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	15.9
Fixed maturities, Total, Fair value	877.4	363.5
Fixed maturities, Total, Gross unrealized losses	26.8	12.5
Corporate debt securities
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	4,287.4	1,578.7
Fixed maturities, Less than twelve months, Gross unrealized losses	141.3	54.4
Fixed maturities, Greater than or equal to twelve months, Fair value	448.6	267.9
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	48.6	40.6
Fixed maturities, Total, Fair value	4,736.0	1,846.6
Fixed maturities, Total, Gross unrealized losses	189.9	95.0
Residential mortgage-backed pass-through securities
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	1,229.4	92.3
Fixed maturities, Less than twelve months, Gross unrealized losses	15.9	0.2
Fixed maturities, Greater than or equal to twelve months, Fair value	21.8	1.6
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	0.6
Fixed maturities, Total, Fair value	1,251.2	93.9
Fixed maturities, Total, Gross unrealized losses	16.5	0.2
Commercial mortgage-backed securities
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	480.6	970.9
Fixed maturities, Less than twelve months, Gross unrealized losses	4.9	22.1
Fixed maturities, Greater than or equal to twelve months, Fair value	321.6	137.4
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	14.1	12.2
Fixed maturities, Total, Fair value	802.2	1,108.3
Fixed maturities, Total, Gross unrealized losses	19.0	34.3
Collateralized debt obligations
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	1,235.9	1,750.6
Fixed maturities, Less than twelve months, Gross unrealized losses	2.9	11.1
Fixed maturities, Greater than or equal to twelve months, Fair value	461.3	931.1
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	7.7	12.9
Fixed maturities, Total, Fair value	1,697.2	2,681.7
Fixed maturities, Total, Gross unrealized losses	10.6	24.0
Other debt obligations
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	2,334.4	802.3
Fixed maturities, Less than twelve months, Gross unrealized losses	17.3	28.1
Fixed maturities, Greater than or equal to twelve months, Fair value	177.4	61.1
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	6.5	1.7
Fixed maturities, Total, Fair value	2,511.8	863.4
Fixed maturities, Total, Gross unrealized losses	23.8	29.8
Principal Life Insurance Company
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	9,056.8	5,638.3
Fixed maturities, Less than twelve months, Gross unrealized losses	136.9	122.4
Fixed maturities, Greater than or equal to twelve months, Fair value	1,736.2	1,391.3
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	95.0	51.5
Fixed maturities, Total, Fair value	10,793.0	7,029.6
Fixed maturities, Total, Gross unrealized losses	$ 231.9	$ 173.9
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)	91.00%	89.00%
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Average Price (percent of carrying value to amortized cost)	98.00%	98.00%
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less Than Twelve Months | item	1,400	619
Available-for-sale Securities in Unrealized Loss Positions, Average Price, Less Than Twelve Months (percent of carrying value to amortized cost)	99.00%	98.00%
Available-for-sale Securities in Unrealized Loss Positions, Percent Investment Grade, Less Than Twelve Months (as a percent)	91.00%	89.00%
Available-for-sale Securities in Unrealized Loss Position, Aggregate Losses On Investment Grade Investments, Less Than Twelve Months	$ 124.8	$ 98.4
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Twelve Months or Longer | item	280	198
Available-for-sale Securities in Unrealized Loss Positions, Average Price, Twelve Months or Longer (percent of carrying value to amortized cost)	95.00%	96.00%
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	$ 25.5	$ 24.9
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)	94.00%	92.00%
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	$ 13.8	$ 11.9
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)	96.00%	92.00%
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	$ 7.7	$ 12.9
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)	98.00%	99.00%