Investments - Gross Unrealized Losses for Fixed Maturities (Details) $ in Millions	Sep. 30, 2020 USD ($) item	Dec. 31, 2019 USD ($) item
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	$ 8,200.8	$ 5,390.1
Fixed maturities, Less than twelve months, Gross unrealized losses	277.3	63.0
Fixed maturities, Greater than or equal to twelve months, Fair value	1,217.5	3,134.9
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	67.8	61.0
Fixed maturities, Total, Fair value	9,418.3	8,525.0
Fixed maturities, Total, Gross unrealized losses	345.1	124.0
U.S. government and agencies
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	102.5	100.0
Fixed maturities, Less than twelve months, Gross unrealized losses	4.0	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	102.5	174.2
Fixed maturities, Total, Gross unrealized losses	4.0	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	366.0	559.9
Fixed maturities, Less than twelve months, Gross unrealized losses	11.7	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	366.0	646.2
Fixed maturities, Total, Gross unrealized losses	11.7	11.6
Corporate debt securities
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	3,221.2	1,041.5
Fixed maturities, Less than twelve months, Gross unrealized losses	165.4	27.8
Fixed maturities, Greater than or equal to twelve months, Fair value	168.6	394.7
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	35.6	24.4
Fixed maturities, Total, Fair value	3,389.8	1,436.2
Fixed maturities, Total, Gross unrealized losses	201.0	52.2
Residential mortgage-backed pass-through securities
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	169.5	429.6
Fixed maturities, Less than twelve months, Gross unrealized losses	0.4	1.4
Fixed maturities, Greater than or equal to twelve months, Fair value	3.1	237.3
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses		2.4
Fixed maturities, Total, Fair value	172.6	666.9
Fixed maturities, Total, Gross unrealized losses	0.4	3.8
Commercial mortgage-backed securities
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	1,173.0	829.3
Fixed maturities, Less than twelve months, Gross unrealized losses	26.2	9.2
Fixed maturities, Greater than or equal to twelve months, Fair value	150.5	268.5
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	12.5	14.8
Fixed maturities, Total, Fair value	1,323.5	1,097.8
Fixed maturities, Total, Gross unrealized losses	38.7	24.0
Collateralized debt obligations
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	2,286.4	639.4
Fixed maturities, Less than twelve months, Gross unrealized losses	28.5	1.8
Fixed maturities, Greater than or equal to twelve months, Fair value	810.2	1,447.8
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	17.1	12.5
Fixed maturities, Total, Fair value	3,096.6	2,087.2
Fixed maturities, Total, Gross unrealized losses	45.6	14.3
Other debt obligations
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	882.2	1,772.8
Fixed maturities, Less than twelve months, Gross unrealized losses	41.1	9.5
Fixed maturities, Greater than or equal to twelve months, Fair value	85.1	613.7
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	2.6	5.4
Fixed maturities, Total, Fair value	967.3	2,386.5
Fixed maturities, Total, Gross unrealized losses	43.7	14.9
Principal Life Insurance Company
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	7,816.7	5,302.7
Fixed maturities, Less than twelve months, Gross unrealized losses	255.3	44.0
Fixed maturities, Greater than or equal to twelve months, Fair value	1,209.5	3,078.0
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	55.8	59.9
Fixed maturities, Total, Fair value	9,026.2	8,380.7
Fixed maturities, Total, Gross unrealized losses	$ 311.1	$ 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)	81.00%	97.00%
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Average Price (percent of carrying value to amortized cost) | item	97	99
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less Than Twelve Months | item	928	882
Available-for-sale Securities in Unrealized Loss Positions, Average Price, Less Than Twelve Months (percent of carrying value to amortized cost) | item	97	99
Available-for-sale Securities in Unrealized Loss Positions, Percent Investment Grade, Less Than Twelve Months (as a percent)	79.00%	98.00%
Available-for-sale Securities in Unrealized Loss Position, Aggregate Losses On Investment Grade Investments, Less Than Twelve Months	$ 163.1	$ 43.1
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Twelve Months or Longer | item	191	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	$ 23.9	$ 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	88	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	$ 16.9
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	98