Nature of Operations and Significant Accounting Policies - Balances and changes in MRBs (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2023	Dec. 31, 2022	Dec. 31, 2021
Balances and changes in MRBs
Balance at beginning of period, market risk benefits	$ (98.2)
Balance at end of period, market risk benefits	60.6	$ (98.2)
Individual variable annuities | Retirement and Income Solutions
Balances and changes in MRBs
Balance at beginning of period, market risk benefits	(72.2)	(494.7)	$ (591.3)
Effect of changes in nonperformance risk at beginning of period	31.7	(109.5)	106.7
Adjusted balance at beginning of period, market risk benefits	(103.9)	(385.2)	(484.6)
Interest accrual and expected policyholder behavior	(57.7)	(90.1)	(120.7)
Benefit payments	0.3	1.3	0.4
Changes in interest rates	126.0	539.1	154.3
Changes in equity markets	76.0	(112.4)	106.4
Changes in equity index volatility	40.4	(50.2)	15.1
Actual policyholder behavior different from expected behavior	(2.8)	1.5	2.3
Changes in future expected policyholder behavior		(6.1)	(96.6)
Changes in other future expected assumptions	(5.3)	(1.8)	38.2
Adjusted balance at end of period, market risk benefits	73.0	(103.9)	(385.2)
Effect of changes in nonperformance risk at end of period	9.3	31.7	(109.5)
Balance at end of period, market risk benefits	82.3	(72.2)	(494.7)
Guaranteed pension | Principal Asset Management | Principal International | Asia
Balances and changes in MRBs
Balance at beginning of period, market risk benefits	(26.0)	(35.1)	(53.3)
Effect of changes in nonperformance risk at beginning of period	(1.2)	(4.3)	6.5
Adjusted balance at beginning of period, market risk benefits	(24.8)	(30.8)	(46.8)
Interest accrual and expected policyholder behavior	(7.6)	(7.1)	(5.3)
Benefit payments	10.4	9.5	5.7
Changes in interest rates	1.1	3.9	7.6
Changes in equity markets	0.2	(1.0)	1.4
Actual policyholder behavior different from expected behavior	(0.1)	0.7	3.6
Changes in future expected policyholder behavior			0.5
Changes in other future expected assumptions			2.3
Foreign currency translation adjustment			0.2
Adjusted balance at end of period, market risk benefits	(20.7)	(24.8)	(30.8)
Effect of changes in nonperformance risk at end of period	(1.0)	(1.2)	(4.3)
Balance at end of period, market risk benefits	$ (21.7)	$ (26.0)	$ (35.1)