Investments - Gross Unrealized Losses for Fixed Maturities (Details) $ in Millions	Dec. 31, 2019 USD ($) item	Dec. 31, 2018 USD ($) item
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	$ 5,390.1	$ 19,116.1
Fixed maturities, Less than twelve months, Gross unrealized losses	63.0	557.7
Fixed maturities, Greater than or equal to twelve months, Fair value	3,134.9	16,608.2
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	61.0	672.0
Fixed maturities, Total, Fair value	8,525.0	35,724.3
Fixed maturities, Total, Gross unrealized losses	124.0	1,229.7
U.S. government and agencies
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	100.0	101.8
Fixed maturities, Less than twelve months, Gross unrealized losses	1.9	1.6
Fixed maturities, Greater than or equal to twelve months, Fair value	74.2	500.3
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	1.1	15.4
Fixed maturities, Total, Fair value	174.2	602.1
Fixed maturities, Total, Gross unrealized losses	3.0	17.0
Non-U.S. governments
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	17.6	210.2
Fixed maturities, Less than twelve months, Gross unrealized losses	0.2	4.7
Fixed maturities, Greater than or equal to twelve months, Fair value	12.4	191.5
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses		9.9
Fixed maturities, Total, Fair value	30.0	401.7
Fixed maturities, Total, Gross unrealized losses	0.2	14.6
States and political subdivisions
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	559.9	1,359.9
Fixed maturities, Less than twelve months, Gross unrealized losses	11.2	33.9
Fixed maturities, Greater than or equal to twelve months, Fair value	86.3	1,590.3
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	0.4	61.4
Fixed maturities, Total, Fair value	646.2	2,950.2
Fixed maturities, Total, Gross unrealized losses	11.6	95.3
Corporate debt securities
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	1,041.5	13,198.4
Fixed maturities, Less than twelve months, Gross unrealized losses	27.8	476.0
Fixed maturities, Greater than or equal to twelve months, Fair value	394.7	6,865.0
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	24.4	369.2
Fixed maturities, Total, Fair value	1,436.2	20,063.4
Fixed maturities, Total, Gross unrealized losses	52.2	845.2
Residential mortgage-backed pass-through securities
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	429.6	236.7
Fixed maturities, Less than twelve months, Gross unrealized losses	1.4	1.0
Fixed maturities, Greater than or equal to twelve months, Fair value	237.3	1,410.2
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	2.4	48.8
Fixed maturities, Total, Fair value	666.9	1,646.9
Fixed maturities, Total, Gross unrealized losses	3.8	49.8
Commercial mortgage-backed securities
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	829.3	790.3
Fixed maturities, Less than twelve months, Gross unrealized losses	9.2	11.6
Fixed maturities, Greater than or equal to twelve months, Fair value	268.5	2,223.2
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	14.8	83.0
Fixed maturities, Total, Fair value	1,097.8	3,013.5
Fixed maturities, Total, Gross unrealized losses	24.0	94.6
Collateralized debt obligations
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	639.4	2,233.3
Fixed maturities, Less than twelve months, Gross unrealized losses	1.8	24.0
Fixed maturities, Greater than or equal to twelve months, Fair value	1,447.8	162.6
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	12.5	6.5
Fixed maturities, Total, Fair value	2,087.2	2,395.9
Fixed maturities, Total, Gross unrealized losses	14.3	30.5
Other debt obligations
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	1,772.8	985.5
Fixed maturities, Less than twelve months, Gross unrealized losses	9.5	4.9
Fixed maturities, Greater than or equal to twelve months, Fair value	613.7	3,665.1
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	5.4	77.8
Fixed maturities, Total, Fair value	2,386.5	4,650.6
Fixed maturities, Total, Gross unrealized losses	14.9	82.7
Principal Life Insurance Company
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	5,302.7	18,764.0
Fixed maturities, Less than twelve months, Gross unrealized losses	44.0	541.3
Fixed maturities, Greater than or equal to twelve months, Fair value	3,078.0	16,287.9
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	59.9	661.6
Fixed maturities, Total, Fair value	8,380.7	35,051.9
Fixed maturities, Total, Gross unrealized losses	$ 103.9	$ 1,202.9
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)	97.00%	95.00%
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Average Price (percent of carrying value to amortized cost) | item	99	97
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less Than Twelve Months | item	882	2,076
Available-for-sale Securities in Unrealized Loss Positions, Average Price, Less Than Twelve Months (percent of carrying value to amortized cost) | item	99	97
Available-for-sale Securities in Unrealized Loss Positions, Percent Investment Grade, Less Than Twelve Months (as a percent)	98.00%	92.00%
Available-for-sale Securities in Unrealized Loss Position, Aggregate Losses On Investment Grade Investments, Less Than Twelve Months	$ 43.1	$ 473.7
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Twelve Months or Longer | item	502	2,335
Available-for-sale Securities in Unrealized Loss Positions, Average Price, Twelve Months or Longer (percent of carrying value to amortized cost) | item	98	96
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	$ 23.6	$ 360.4
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) | item	94	95
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	$ 82.2
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	96