Investments - Gross Unrealized Losses for Fixed Maturities (Details) $ in Millions	Dec. 31, 2020 USD ($) item $ / shares	Dec. 31, 2019 USD ($) item
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	$ 5,909.4	$ 5,390.1
Fixed maturities, Less than twelve months, Gross unrealized losses	138.8	63.0
Fixed maturities, Greater than or equal to twelve months, Fair value	1,399.1	3,134.9
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	67.4	61.0
Fixed maturities, Total, Fair value	7,308.5	8,525.0
Fixed maturities, Total, Gross unrealized losses	206.2	124.0
U.S. government and agencies
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	351.1	100.0
Fixed maturities, Less than twelve months, Gross unrealized losses	10.4	1.9
Fixed maturities, Greater than or equal to twelve months, Fair value		74.2
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses		1.1
Fixed maturities, Total, Fair value	351.1	174.2
Fixed maturities, Total, Gross unrealized losses	10.4	3.0
Non-U.S. governments
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value		17.6
Fixed maturities, Less than twelve months, Gross unrealized losses		0.2
Fixed maturities, Greater than or equal to twelve months, Fair value		12.4
Fixed maturities, Total, Fair value		30.0
Fixed maturities, Total, Gross unrealized losses		0.2
States and political subdivisions
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	363.5	559.9
Fixed maturities, Less than twelve months, Gross unrealized losses	12.5	11.2
Fixed maturities, Greater than or equal to twelve months, Fair value		86.3
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses		0.4
Fixed maturities, Total, Fair value	363.5	646.2
Fixed maturities, Total, Gross unrealized losses	12.5	11.6
Corporate debt securities
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	1,578.7	1,041.5
Fixed maturities, Less than twelve months, Gross unrealized losses	54.4	27.8
Fixed maturities, Greater than or equal to twelve months, Fair value	267.9	394.7
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	40.6	24.4
Fixed maturities, Total, Fair value	1,846.6	1,436.2
Fixed maturities, Total, Gross unrealized losses	95.0	52.2
Residential mortgage-backed pass-through securities
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	92.3	429.6
Fixed maturities, Less than twelve months, Gross unrealized losses	0.2	1.4
Fixed maturities, Greater than or equal to twelve months, Fair value	1.6	237.3
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses		2.4
Fixed maturities, Total, Fair value	93.9	666.9
Fixed maturities, Total, Gross unrealized losses	0.2	3.8
Commercial mortgage-backed securities
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	970.9	829.3
Fixed maturities, Less than twelve months, Gross unrealized losses	22.1	9.2
Fixed maturities, Greater than or equal to twelve months, Fair value	137.4	268.5
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	12.2	14.8
Fixed maturities, Total, Fair value	1,108.3	1,097.8
Fixed maturities, Total, Gross unrealized losses	34.3	24.0
Collateralized debt obligations
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	1,750.6	639.4
Fixed maturities, Less than twelve months, Gross unrealized losses	11.1	1.8
Fixed maturities, Greater than or equal to twelve months, Fair value	931.1	1,447.8
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	12.9	12.5
Fixed maturities, Total, Fair value	2,681.7	2,087.2
Fixed maturities, Total, Gross unrealized losses	24.0	14.3
Other debt obligations
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	802.3	1,772.8
Fixed maturities, Less than twelve months, Gross unrealized losses	28.1	9.5
Fixed maturities, Greater than or equal to twelve months, Fair value	61.1	613.7
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	1.7	5.4
Fixed maturities, Total, Fair value	863.4	2,386.5
Fixed maturities, Total, Gross unrealized losses	29.8	14.9
Principal Life Insurance Company
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	5,638.3	5,302.7
Fixed maturities, Less than twelve months, Gross unrealized losses	122.4	44.0
Fixed maturities, Greater than or equal to twelve months, Fair value	1,391.3	3,078.0
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	51.5	59.9
Fixed maturities, Total, Fair value	7,029.6	8,380.7
Fixed maturities, Total, Gross unrealized losses	$ 173.9	$ 103.9
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)	89.00%	97.00%
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Average Price (percent of carrying value to amortized cost)	98	99
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less Than Twelve Months | item	619	882
Available-for-sale Securities in Unrealized Loss Positions, Average Price, Less Than Twelve Months (percent of carrying value to amortized cost)	98	99
Available-for-sale Securities in Unrealized Loss Positions, Percent Investment Grade, Less Than Twelve Months (as a percent)	89.00%	98.00%
Available-for-sale Securities in Unrealized Loss Position, Aggregate Losses On Investment Grade Investments, Less Than Twelve Months	$ 98.4	$ 43.1
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Twelve Months or Longer | item	198	502
Available-for-sale Securities in Unrealized Loss Positions, Average Price, Twelve Months or Longer (percent of carrying value to amortized cost) | item	96	98
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	$ 24.9	$ 23.6
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) | item	92	94
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		$ 14.6
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) | item		95
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	$ 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) | item	99