Revenues from Contracts with Customers - Disaggregation within Segments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Revenues from Contracts with Customers - Disaggregation of Revenues
Total fees and other revenues	$ 1,251.8	$ 1,143.0	$ 3,711.0	$ 3,332.6
Premiums and other considerations	1,230.5	1,184.3	3,335.3	4,628.4
Net investment income (loss)	1,093.4	917.9	3,167.0	2,846.1
Operating Segments
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	796.5	681.9	2,329.8	2,035.6
Total operating revenues	3,622.0	3,292.4	10,349.3	10,903.1
Operating Segments | Retirement and Income Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	146.6	135.5	441.0	447.2
Total operating revenues	1,652.9	1,579.9	4,547.8	5,665.6
Operating Segments | Retirement and Income Solutions | Retirement and Income Solutions - Fee
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	144.2	133.4	433.8	441.0
Fees and other revenues not within the scope of revenue recognition guidance	324.1	293.5	964.3	829.1
Total fees and other revenues	468.3	426.9	1,398.1	1,270.1
Premiums and other considerations		0.7	0.5	3.4
Net investment income (loss)	112.6	104.0	325.3	310.6
Total operating revenues	580.9	531.6	1,723.9	1,584.1
Operating Segments | Retirement and Income Solutions | Retirement and Income Solutions - Fee | Administrative service fee revenue
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	144.2	132.8	431.4	439.7
Operating Segments | Retirement and Income Solutions | Retirement and Income Solutions - Fee | Other fee revenue
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers		0.6	2.4	1.3
Operating Segments | Retirement and Income Solutions | Retirement and Income Solutions - Spread
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	2.4	2.1	7.2	6.2
Fees and other revenues not within the scope of revenue recognition guidance	2.1	1.9	6.8	7.8
Total fees and other revenues	4.5	4.0	14.0	14.0
Premiums and other considerations	482.1	517.1	1,111.9	2,497.7
Net investment income (loss)	585.4	527.2	1,698.0	1,569.8
Total operating revenues	1,072.0	1,048.3	2,823.9	4,081.5
Operating Segments | Retirement and Income Solutions | Retirement and Income Solutions - Spread | Deposit account fee revenue
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	2.2	2.1	6.7	6.2
Operating Segments | Retirement and Income Solutions | Retirement and Income Solutions - Spread | Commission income
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	0.2		0.5
Operating Segments | Principal Global Investors
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	453.5	377.0	1,303.6	1,108.1
Fees and other revenues not within the scope of revenue recognition guidance	11.0	4.8	27.1	14.2
Total fees and other revenues	464.5	381.8	1,330.7	1,122.3
Net investment income (loss)	0.7	1.0	3.4	4.2
Total operating revenues	465.2	382.8	1,334.1	1,126.5
Operating Segments | Principal Global Investors | Management fee revenue
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	389.9	330.2	1,119.9	954.7
Operating Segments | Principal Global Investors | Other fee revenue
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	63.6	46.8	183.7	153.4
Operating Segments | Principal International
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	125.1	111.1	373.5	322.2
Fees and other revenues not within the scope of revenue recognition guidance	1.0	1.4	3.1	4.2
Total fees and other revenues	126.1	112.5	376.6	326.4
Premiums and other considerations	36.6	16.6	105.8	130.2
Net investment income (loss)	165.4	96.2	459.9	345.6
Total operating revenues	328.1	225.3	942.3	802.2
Operating Segments | Principal International | Latin America
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	91.5	80.7	273.3	236.5
Operating Segments | Principal International | Asia
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	33.5	30.5	99.3	85.8
Operating Segments | Principal International | Principal International Corporate / Regional Office
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	0.5	0.2	2.0	0.7
Operating Segments | Principal International | Geographical Eliminations
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	(0.4)	(0.3)	(1.1)	(0.8)
Operating Segments | Principal International | Management fee revenue
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	123.0	110.1	367.8	319.3
Operating Segments | Principal International | Other fee revenue
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	2.1	1.0	5.7	2.9
Operating Segments | U.S. Insurance Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	19.7	15.9	55.8	46.9
Total operating revenues	1,190.1	1,121.4	3,546.9	3,332.8
Operating Segments | U.S. Insurance Solutions | Specialty Benefits insurance
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	3.9	3.9	11.4	11.2
Fees and other revenues not within the scope of revenue recognition guidance	5.4	4.9	14.9	14.4
Total fees and other revenues	9.3	8.8	26.3	25.6
Premiums and other considerations	629.6	570.9	1,851.7	1,741.8
Net investment income (loss)	45.5	38.4	129.9	116.7
Total operating revenues	684.4	618.1	2,007.9	1,884.1
Operating Segments | U.S. Insurance Solutions | Specialty Benefits insurance | Administrative service fee revenue
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	3.9	3.9	11.4	11.2
Operating Segments | U.S. Insurance Solutions | Individual Life insurance
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	15.8	12.0	44.5	35.8
Fees and other revenues not within the scope of revenue recognition guidance	204.1	232.6	635.8	634.4
Total fees and other revenues	219.9	244.6	680.3	670.2
Premiums and other considerations	82.2	79.0	265.4	255.3
Net investment income (loss)	203.6	179.7	593.4	523.3
Total operating revenues	505.7	503.3	1,539.1	1,448.8
Operating Segments | U.S. Insurance Solutions | Individual Life insurance | Administrative service fee revenue
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	6.8	5.4	19.3	16.1
Operating Segments | U.S. Insurance Solutions | Individual Life insurance | Commission income
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	9.0	6.6	25.2	19.7
Operating Segments | Corporate
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	51.6	42.4	155.9	111.2
Fees and other revenues not within the scope of revenue recognition guidance	(91.8)	(75.8)	(266.3)	(221.1)
Total fees and other revenues	(40.2)	(33.4)	(110.4)	(109.9)
Net investment income (loss)	25.9	16.4	88.6	85.9
Total operating revenues	(14.3)	(17.0)	(21.8)	(24.0)
Operating Segments | Corporate | Other fee revenue
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	17.3	13.6	51.0	38.3
Operating Segments | Corporate | Commission income
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	99.8	78.0	287.5	231.0
Operating Segments | Corporate | Revenue by type - Eliminations
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	$ (65.5)	$ (49.2)	$ (182.6)	$ (158.1)