Investments - Gross Unrealized Losses for Fixed Maturities (Details) $ in Millions	Jun. 30, 2020 USD ($) item	Dec. 31, 2019 USD ($) item
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	$ 10,186.8	$ 5,390.1
Fixed maturities, Less than twelve months, Gross unrealized losses	535.1	63.0
Fixed maturities, Greater than or equal to twelve months, Fair value	1,182.9	3,134.9
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	73.0	61.0
Fixed maturities, Total, Fair value	11,369.7	8,525.0
Fixed maturities, Total, Gross unrealized losses	608.1	124.0
U.S. government and agencies
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	30.1	100.0
Fixed maturities, Less than twelve months, Gross unrealized losses	0.2	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	30.1	174.2
Fixed maturities, Total, Gross unrealized losses	0.2	3.0
Non-U.S. governments
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	2.6	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	2.6	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	460.5	559.9
Fixed maturities, Less than twelve months, Gross unrealized losses	18.1	11.2
Fixed maturities, Greater than or equal to twelve months, Fair value	21.0	86.3
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses		0.4
Fixed maturities, Total, Fair value	481.5	646.2
Fixed maturities, Total, Gross unrealized losses	18.1	11.6
Corporate debt securities
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	3,521.5	1,041.5
Fixed maturities, Less than twelve months, Gross unrealized losses	248.4	27.8
Fixed maturities, Greater than or equal to twelve months, Fair value	149.7	394.7
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	26.8	24.4
Fixed maturities, Total, Fair value	3,671.2	1,436.2
Fixed maturities, Total, Gross unrealized losses	275.2	52.2
Residential mortgage-backed pass-through securities
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	195.0	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	3.2	237.3
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses		2.4
Fixed maturities, Total, Fair value	198.2	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	2,507.8	829.3
Fixed maturities, Less than twelve months, Gross unrealized losses	134.3	9.2
Fixed maturities, Greater than or equal to twelve months, Fair value	151.3	268.5
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	15.5	14.8
Fixed maturities, Total, Fair value	2,659.1	1,097.8
Fixed maturities, Total, Gross unrealized losses	149.8	24.0
Collateralized debt obligations
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	2,483.8	639.4
Fixed maturities, Less than twelve months, Gross unrealized losses	62.2	1.8
Fixed maturities, Greater than or equal to twelve months, Fair value	776.7	1,447.8
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	27.3	12.5
Fixed maturities, Total, Fair value	3,260.5	2,087.2
Fixed maturities, Total, Gross unrealized losses	89.5	14.3
Other debt obligations
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	985.5	1,772.8
Fixed maturities, Less than twelve months, Gross unrealized losses	71.5	9.5
Fixed maturities, Greater than or equal to twelve months, Fair value	81.0	613.7
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	3.4	5.4
Fixed maturities, Total, Fair value	1,066.5	2,386.5
Fixed maturities, Total, Gross unrealized losses	74.9	14.9
Principal Life Insurance Company
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	9,843.7	5,302.7
Fixed maturities, Less than twelve months, Gross unrealized losses	502.1	44.0
Fixed maturities, Greater than or equal to twelve months, Fair value	1,175.2	3,078.0
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	72.5	59.9
Fixed maturities, Total, Fair value	11,018.9	8,380.7
Fixed maturities, Total, Gross unrealized losses	$ 574.6	$ 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	95	99
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less Than Twelve Months | item	1,286	882
Available-for-sale Securities in Unrealized Loss Positions, Average Price, Less Than Twelve Months (percent of carrying value to amortized cost) | item	95	99
Available-for-sale Securities in Unrealized Loss Positions, Percent Investment Grade, Less Than Twelve Months (as a percent)	80.00%	98.00%
Available-for-sale Securities in Unrealized Loss Position, Aggregate Losses On Investment Grade Investments, Less Than Twelve Months	$ 347.5	$ 43.1
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Twelve Months or Longer | item	196	502
Available-for-sale Securities in Unrealized Loss Positions, Average Price, Twelve Months or Longer (percent of carrying value to amortized cost) | item	94	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	$ 26.8	$ 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	85	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	$ 27.1
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) | item	97