Revenues from Contracts with Customers - Disaggregation within Segments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Revenues from Contracts with Customers - Disaggregation of Revenues
Total fees and other revenues	$ 1,044.3	$ 1,006.6	$ 2,189.6	$ 1,980.1
Premiums and other considerations	1,153.1	1,932.9	3,444.1	3,657.8
Net investment income (loss)	944.6	1,037.1	1,928.2	1,998.1
Operating Segments
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	653.3	587.2	1,353.7	1,148.4
Total operating revenues	3,181.1	4,002.2	7,610.7	7,682.9
Operating Segments | Retirement and Income Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	153.2	66.2	311.7	127.5
Total operating revenues	1,480.0	2,159.6	4,085.7	4,051.5
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	151.2	63.9	307.6	122.3
Fees and other revenues not within the scope of revenue recognition guidance	255.1	281.6	535.6	553.4
Total fees and other revenues	406.3	345.5	843.2	675.7
Premiums and other considerations	0.9	0.6	2.7	1.7
Net investment income (loss)	102.2	104.4	206.6	203.9
Total operating revenues	509.4	450.5	1,052.5	881.3
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	150.7	63.3	306.9	121.5
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	0.5	0.6	0.7	0.8
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	2.0	2.3	4.1	5.2
Fees and other revenues not within the scope of revenue recognition guidance	2.8	4.2	5.9	9.1
Total fees and other revenues	4.8	6.5	10.0	14.3
Premiums and other considerations	459.5	1,171.5	1,980.6	2,129.0
Net investment income (loss)	506.3	531.1	1,042.6	1,026.9
Total operating revenues	970.6	1,709.1	3,033.2	3,170.2
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	2.0	2.3	4.1	5.2
Operating Segments | Principal Global Investors
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	348.2	346.2	731.1	678.5
Fees and other revenues not within the scope of revenue recognition guidance	3.9	10.4	9.4	14.7
Total fees and other revenues	352.1	356.6	740.5	693.2
Net investment income (loss)	0.8	2.9	3.2	5.3
Total operating revenues	352.9	359.5	743.7	698.5
Operating Segments | Principal Global Investors | Management fee revenue
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	303.5	304.2	624.5	597.0
Operating Segments | Principal Global Investors | Other fee revenue
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	44.7	42.0	106.6	81.5
Operating Segments | Principal International
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	101.6	116.2	211.1	231.1
Fees and other revenues not within the scope of revenue recognition guidance	1.5	1.7	2.8	3.4
Total fees and other revenues	103.1	117.9	213.9	234.5
Premiums and other considerations	28.6	109.5	113.6	227.0
Net investment income (loss)	144.7	184.2	249.4	330.6
Total operating revenues	276.4	411.6	576.9	792.1
Operating Segments | Principal International | Latin America
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	74.3	88.6	155.8	176.7
Operating Segments | Principal International | Asia
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	27.3	27.6	55.3	54.6
Operating Segments | Principal International | Principal International Corporate / Regional Office
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	0.3	0.2	0.5	0.4
Operating Segments | Principal International | Geographical Eliminations
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	(0.3)	(0.2)	(0.5)	(0.6)
Operating Segments | Principal International | Management fee revenue
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	100.7	115.5	209.2	229.8
Operating Segments | Principal International | Other fee revenue
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	0.9	0.7	1.9	1.3
Operating Segments | U.S. Insurance Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	14.4	16.1	31.0	31.8
Total operating revenues	1,088.1	1,087.3	2,211.4	2,155.4
Operating Segments | U.S. Insurance Solutions | Specialty Benefits insurance
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	3.5	3.6	7.3	7.3
Fees and other revenues not within the scope of revenue recognition guidance	4.6	4.9	9.5	9.8
Total fees and other revenues	8.1	8.5	16.8	17.1
Premiums and other considerations	582.5	571.6	1,170.9	1,127.5
Net investment income (loss)	36.4	45.7	78.3	83.0
Total operating revenues	627.0	625.8	1,266.0	1,227.6
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	3.5	3.6	7.3	7.3
Operating Segments | U.S. Insurance Solutions | Individual Life insurance
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	11.0	12.6	23.8	24.6
Fees and other revenues not within the scope of revenue recognition guidance	191.8	190.3	401.8	382.5
Total fees and other revenues	202.8	202.9	425.6	407.1
Premiums and other considerations	81.6	79.7	176.3	172.6
Net investment income (loss)	176.8	179.0	343.6	348.2
Total operating revenues	461.2	461.6	945.5	927.9
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	5.0	6.2	10.7	12.1
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	6.0	6.4	13.1	12.5
Operating Segments | Corporate
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	35.9	42.5	68.8	79.5
Fees and other revenues not within the scope of revenue recognition guidance	(69.2)	(77.9)	(145.3)	(148.5)
Total fees and other revenues	(33.3)	(35.4)	(76.5)	(69.0)
Net investment income (loss)	17.0	19.6	69.5	54.4
Total operating revenues	(16.3)	(15.8)	(7.0)	(14.6)
Operating Segments | Corporate | Other fee revenue
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	13.6	11.2	24.7	20.0
Operating Segments | Corporate | Commission income
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	72.6	78.7	153.0	154.5
Operating Segments | Corporate | Revenue by type - Eliminations
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	$ (50.3)	$ (47.4)	$ (108.9)	$ (95.0)