Investments - Gross Unrealized Losses for Fixed Maturities (Details) $ in Millions	Jun. 30, 2019 USD ($) item	Dec. 31, 2018 USD ($) item
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	$ 1,815.0	$ 19,116.1
Fixed maturities, Less than twelve months, Gross unrealized losses	15.2	557.7
Fixed maturities, Greater than or equal to twelve months, Fair value	7,388.4	16,608.2
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	136.0	672.0
Fixed maturities, Total, Fair value	9,203.4	35,724.3
Fixed maturities, Total, Gross unrealized losses	151.2	1,229.7
U.S. government and agencies
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	5.7	101.8
Fixed maturities, Less than twelve months, Gross unrealized losses		1.6
Fixed maturities, Greater than or equal to twelve months, Fair value	160.8	500.3
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	3.9	15.4
Fixed maturities, Total, Fair value	166.5	602.1
Fixed maturities, Total, Gross unrealized losses	3.9	17.0
Non-U.S. governments
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	13.2	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	47.9	191.5
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	2.3	9.9
Fixed maturities, Total, Fair value	61.1	401.7
Fixed maturities, Total, Gross unrealized losses	2.5	14.6
States and political subdivisions
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	2.3	1,359.9
Fixed maturities, Less than twelve months, Gross unrealized losses		33.9
Fixed maturities, Greater than or equal to twelve months, Fair value	509.8	1,590.3
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	5.6	61.4
Fixed maturities, Total, Fair value	512.1	2,950.2
Fixed maturities, Total, Gross unrealized losses	5.6	95.3
Corporate debt securities
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	579.1	13,198.4
Fixed maturities, Less than twelve months, Gross unrealized losses	9.7	476.0
Fixed maturities, Greater than or equal to twelve months, Fair value	2,556.0	6,865.0
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	71.6	369.2
Fixed maturities, Total, Fair value	3,135.1	20,063.4
Fixed maturities, Total, Gross unrealized losses	81.3	845.2
Residential mortgage-backed pass-through securities
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	1.0	236.7
Fixed maturities, Less than twelve months, Gross unrealized losses		1.0
Fixed maturities, Greater than or equal to twelve months, Fair value	747.4	1,410.2
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	8.8	48.8
Fixed maturities, Total, Fair value	748.4	1,646.9
Fixed maturities, Total, Gross unrealized losses	8.8	49.8
Commercial mortgage-backed securities
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	127.3	790.3
Fixed maturities, Less than twelve months, Gross unrealized losses	2.1	11.6
Fixed maturities, Greater than or equal to twelve months, Fair value	625.6	2,223.2
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	19.0	83.0
Fixed maturities, Total, Fair value	752.9	3,013.5
Fixed maturities, Total, Gross unrealized losses	21.1	94.6
Collateralized debt obligations
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	975.5	2,233.3
Fixed maturities, Less than twelve months, Gross unrealized losses	2.7	24.0
Fixed maturities, Greater than or equal to twelve months, Fair value	1,063.9	162.6
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	10.9	6.5
Fixed maturities, Total, Fair value	2,039.4	2,395.9
Fixed maturities, Total, Gross unrealized losses	13.6	30.5
Other debt obligations
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	110.9	985.5
Fixed maturities, Less than twelve months, Gross unrealized losses	0.5	4.9
Fixed maturities, Greater than or equal to twelve months, Fair value	1,677.0	3,665.1
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	13.9	77.8
Fixed maturities, Total, Fair value	1,787.9	4,650.6
Fixed maturities, Total, Gross unrealized losses	14.4	82.7
Principal Life Insurance Company
Gross Unrealized Losses for Fixed Maturities
Fixed maturities, Less than twelve months, Fair value	1,768.2	18,764.0
Fixed maturities, Less than twelve months, Gross unrealized losses	14.3	541.3
Fixed maturities, Greater than or equal to twelve months, Fair value	7,304.3	16,287.9
Fixed maturities, Greater than or equal to twelve months, Gross unrealized losses	133.8	661.6
Fixed maturities, Total, Fair value	9,072.5	35,051.9
Fixed maturities, Total, Gross unrealized losses	$ 148.1	$ 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)	94.00%	95.00%
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Average Price (percent of carrying value to amortized cost) | item	98	97
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less Than Twelve Months | item	177	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)	84.00%	92.00%
Available-for-sale Securities in Unrealized Loss Position, Aggregate Losses On Investment Grade Investments, Less Than Twelve Months	$ 8.8	$ 473.7
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Twelve Months or Longer | item	1,081	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	$ 69.7	$ 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	97	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	$ 18.8	$ 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	97	96