Revenues from Contracts with Customers - Disaggregation within Segments (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Revenues from Contracts with Customers - Disaggregation of Revenues
Total fees and other revenues	$ 1,145.3	$ 973.5
Premiums and other considerations	2,291.0	1,724.9
Net investment income (loss)	983.6	961.0
Operating Segments
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	700.4	561.2
Total operating revenues	4,429.6	3,680.7
Operating Segments | Retirement and Income Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	158.5	61.3
Total operating revenues	2,605.7	1,891.9
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	156.4	58.4
Fees and other revenues not within the scope of revenue recognition guidance	280.5	271.8
Total fees and other revenues	436.9	330.2
Premiums and other considerations	1.8	1.1
Net investment income (loss)	104.4	99.5
Total operating revenues	543.1	430.8
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	156.2	58.2
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	0.2	0.2
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	2.1	2.9
Fees and other revenues not within the scope of revenue recognition guidance	3.1	4.9
Total fees and other revenues	5.2	7.8
Premiums and other considerations	1,521.1	957.5
Net investment income (loss)	536.3	495.8
Total operating revenues	2,062.6	1,461.1
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	2.1	2.9
Operating Segments | Principal Global Investors
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	382.9	332.3
Fees and other revenues not within the scope of revenue recognition guidance	5.5	4.3
Total fees and other revenues	388.4	336.6
Net investment income (loss)	2.4	2.4
Total operating revenues	390.8	339.0
Operating Segments | Principal Global Investors | Management fee revenue
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	321.0	292.8
Operating Segments | Principal Global Investors | Other fee revenue
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	61.9	39.5
Operating Segments | Principal International
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	109.5	114.9
Fees and other revenues not within the scope of revenue recognition guidance	1.3	1.7
Total fees and other revenues	110.8	116.6
Premiums and other considerations	85.0	117.5
Net investment income (loss)	104.7	146.4
Total operating revenues	300.5	380.5
Operating Segments | Principal International | Latin America
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	81.5	88.1
Operating Segments | Principal International | Asia
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	28.0	27.0
Operating Segments | Principal International | Principal International Corporate / Regional Office
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	0.2	0.2
Operating Segments | Principal International | Geographical Eliminations
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	(0.2)	(0.4)
Operating Segments | Principal International | Management fee revenue
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	108.5	114.3
Operating Segments | Principal International | Other fee revenue
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	1.0	0.6
Operating Segments | U.S. Insurance Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	16.6	15.7
Total operating revenues	1,123.3	1,068.1
Operating Segments | U.S. Insurance Solutions | Specialty Benefits insurance
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	3.8	3.7
Fees and other revenues not within the scope of revenue recognition guidance	4.9	4.9
Total fees and other revenues	8.7	8.6
Premiums and other considerations	588.4	555.9
Net investment income (loss)	41.9	37.3
Total operating revenues	639.0	601.8
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	3.8	3.7
Operating Segments | U.S. Insurance Solutions | Individual Life insurance
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	12.8	12.0
Fees and other revenues not within the scope of revenue recognition guidance	210.0	192.2
Total fees and other revenues	222.8	204.2
Premiums and other considerations	94.7	92.9
Net investment income (loss)	166.8	169.2
Total operating revenues	484.3	466.3
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	5.7	5.9
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	7.1	6.1
Operating Segments | Corporate
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	32.9	37.0
Fees and other revenues not within the scope of revenue recognition guidance	(76.1)	(70.6)
Total fees and other revenues	(43.2)	(33.6)
Net investment income (loss)	52.5	34.8
Total operating revenues	9.3	1.2
Operating Segments | Corporate | Other fee revenue
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	11.1	8.8
Operating Segments | Corporate | Commission income
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	80.4	75.8
Operating Segments | Corporate | Revenue by type - Eliminations
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	$ (58.6)	$ (47.6)