Segment Information - Reconciliation of Segment Operating Revenues and Earnings to Consolidated (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Operating Revenue And Profit (Loss) From Segments To Consolidated
Net realized capital gains (losses), net of related revenue adjustments	$ (188.2)	$ 31.6	$ (157.0)	$ 105.4
Total revenues	3,423.7	3,310.7	10,171.6	10,976.6
Pre-tax net realized capital gains (losses), as adjusted	(133.5)	9.9	(141.7)	(41.1)
Income (loss) before income taxes	428.1	275.1	1,474.6	1,101.9
Retirement and Income Solutions
Operating Revenue And Profit (Loss) From Segments To Consolidated
Inter-segment revenues	106.0	87.5	307.7	249.1
Principal Global Investors
Operating Revenue And Profit (Loss) From Segments To Consolidated
Inter-segment revenues	79.7	65.6	223.9	200.7
Operating Segments
Operating Revenue And Profit (Loss) From Segments To Consolidated
Operating revenues	3,622.0	3,292.4	10,349.3	10,903.1
Pre-tax operating earnings (losses)	568.8	276.9	1,630.6	1,150.7
Operating Segments | Retirement and Income Solutions
Operating Revenue And Profit (Loss) From Segments To Consolidated
Operating revenues	1,652.9	1,579.9	4,547.8	5,665.6
Pre-tax operating earnings (losses)	243.2	281.1	811.4	700.1
Operating Segments | Retirement and Income Solutions | Retirement and Income Solutions - Fee
Operating Revenue And Profit (Loss) From Segments To Consolidated
Operating revenues	580.9	531.6	1,723.9	1,584.1
Operating Segments | Retirement and Income Solutions | Retirement and Income Solutions - Spread
Operating Revenue And Profit (Loss) From Segments To Consolidated
Operating revenues	1,072.0	1,048.3	2,823.9	4,081.5
Operating Segments | Principal Global Investors
Operating Revenue And Profit (Loss) From Segments To Consolidated
Operating revenues	465.2	382.8	1,334.1	1,126.5
Pre-tax operating earnings (losses)	190.1	140.9	515.6	361.0
Operating Segments | Principal International
Operating Revenue And Profit (Loss) From Segments To Consolidated
Operating revenues	328.1	225.3	942.3	802.2
Pre-tax operating earnings (losses)	81.0	58.7	203.8	184.0
Operating Segments | U.S. Insurance Solutions
Operating Revenue And Profit (Loss) From Segments To Consolidated
Operating revenues	1,190.1	1,121.4	3,546.9	3,332.8
Pre-tax operating earnings (losses)	151.6	(134.1)	373.4	150.4
Operating Segments | U.S. Insurance Solutions | Specialty Benefits insurance
Operating Revenue And Profit (Loss) From Segments To Consolidated
Operating revenues	684.4	618.1	2,007.9	1,884.1
Operating Segments | U.S. Insurance Solutions | Individual Life insurance
Operating Revenue And Profit (Loss) From Segments To Consolidated
Operating revenues	505.7	503.3	1,539.1	1,448.8
Operating Segments | U.S. Insurance Solutions | Eliminations
Operating Revenue And Profit (Loss) From Segments To Consolidated
Operating revenues			(0.1)	(0.1)
Operating Segments | Corporate
Operating Revenue And Profit (Loss) From Segments To Consolidated
Operating revenues	(14.3)	(17.0)	(21.8)	(24.0)
Pre-tax operating earnings (losses)	(97.1)	(69.7)	(273.6)	(244.8)
Reconciling Items
Operating Revenue And Profit (Loss) From Segments To Consolidated
Net realized capital gains (losses), net of related revenue adjustments	(188.2)	31.6	(157.0)	105.4
Adjustments related to equity method investments	(10.1)	(13.3)	(20.7)	(31.9)
Pre-tax net realized capital gains (losses), as adjusted	(133.5)	9.9	(141.7)	(41.1)
Earnings adjustments related to equity method investments and noncontrolling interest	$ (7.2)	$ (11.7)	$ (14.3)	$ (7.7)