Revenues from Contracts with Customers - Disaggregation within Segments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Revenues from Contracts with Customers - Disaggregation of Revenues
Total fees and other revenues	$ 1,006.6	$ 1,012.6	$ 1,980.1	$ 2,023.9
Premiums and other considerations	1,932.9	1,265.8	3,657.8	2,261.0
Net investment income (loss)	1,037.1	889.1	1,998.1	1,791.3
Operating Segments
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	587.2	614.3	1,148.4	1,217.1
Total operating revenues	4,002.2	3,206.2	7,682.9	6,153.4
Operating Segments | Retirement and Income Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	66.2	69.6	127.5	134.0
Total operating revenues	2,159.6	1,449.9	4,051.5	2,628.8
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	63.9	66.9	122.3	128.6
Fees and other revenues not within the scope of revenue recognition guidance	281.6	282.8	553.4	569.2
Total fees and other revenues	345.5	349.7	675.7	697.8
Premiums and other considerations	0.6	0.9	1.7	2.4
Net investment income (loss)	104.4	95.9	203.9	194.1
Total operating revenues	450.5	446.5	881.3	894.3
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	63.3	66.3	121.5	127.9
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	0.6	0.8	0.7
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	2.3	2.7	5.2	5.4
Fees and other revenues not within the scope of revenue recognition guidance	4.2	2.9	9.1	5.6
Total fees and other revenues	6.5	5.6	14.3	11.0
Premiums and other considerations	1,171.5	555.1	2,129.0	843.4
Net investment income (loss)	531.1	442.7	1,026.9	880.1
Total operating revenues	1,709.1	1,003.4	3,170.2	1,734.5
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	2.3	2.7	5.2	5.4
Operating Segments | Principal Global Investors
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	346.2	371.1	678.5	731.4
Fees and other revenues not within the scope of revenue recognition guidance	10.4	7.3	14.7	14.0
Total fees and other revenues	356.6	378.4	693.2	745.4
Net investment income (loss)	2.9	1.2	5.3	2.7
Total operating revenues	359.5	379.6	698.5	748.1
Operating Segments | Principal Global Investors | Management fee revenue
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	304.2	312.2	597.0	628.5
Operating Segments | Principal Global Investors | Other fee revenue
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	42.0	58.9	81.5	102.9
Operating Segments | Principal International
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	116.2	123.1	231.1	244.2
Fees and other revenues not within the scope of revenue recognition guidance	1.7	1.8	3.4	3.8
Total fees and other revenues	117.9	124.9	234.5	248.0
Premiums and other considerations	109.5	101.6	227.0	199.5
Net investment income (loss)	184.2	163.1	330.6	325.2
Total operating revenues	411.6	389.6	792.1	772.7
Operating Segments | Principal International | Latin America
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	88.6	96.5	176.7	189.0
Operating Segments | Principal International | Asia
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	27.6	26.7	54.6	55.4
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	0.3	0.4	0.6
Operating Segments | Principal International | Geographical Eliminations
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	(0.2)	(0.4)	(0.6)	(0.8)
Operating Segments | Principal International | Management fee revenue
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	115.5	121.8	229.8	241.9
Operating Segments | Principal International | Other fee revenue
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	0.7	1.3	1.3	2.3
Operating Segments | U.S. Insurance Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	16.1	13.5	31.8	26.6
Total operating revenues	1,087.3	1,004.1	2,155.4	2,020.8
Operating Segments | U.S. Insurance Solutions | Specialty Benefits insurance
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	3.6	3.8	7.3	7.5
Fees and other revenues not within the scope of revenue recognition guidance	4.9	5.2	9.8	10.4
Total fees and other revenues	8.5	9.0	17.1	17.9
Premiums and other considerations	571.6	527.7	1,127.5	1,044.7
Net investment income (loss)	45.7	35.5	83.0	75.0
Total operating revenues	625.8	572.2	1,227.6	1,137.6
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	3.6	3.8	7.3	7.5
Operating Segments | U.S. Insurance Solutions | Individual Life insurance
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	12.6	9.8	24.6	19.2
Fees and other revenues not within the scope of revenue recognition guidance	190.3	177.7	382.5	363.4
Total fees and other revenues	202.9	187.5	407.1	382.6
Premiums and other considerations	79.7	80.5	172.6	171.0
Net investment income (loss)	179.0	164.0	348.2	329.7
Total operating revenues	461.6	432.0	927.9	883.3
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	6.2	5.8	12.1	11.6
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	6.4	4.0	12.5	7.6
Operating Segments | U.S. Insurance Solutions | U.S. Insurance Solutions Eliminations
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	(0.1)	(0.1)	(0.1)	(0.1)
Total operating revenues	(0.1)	(0.1)	(0.1)	(0.1)
Operating Segments | Corporate
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	42.5	37.0	79.5	80.9
Fees and other revenues not within the scope of revenue recognition guidance	(77.9)	(78.7)	(148.5)	(157.3)
Total fees and other revenues	(35.4)	(41.7)	(69.0)	(76.4)
Net investment income (loss)	19.6	24.7	54.4	59.4
Total operating revenues	(15.8)	(17.0)	(14.6)	(17.0)
Operating Segments | Corporate | Other fee revenue
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	11.2	9.2	20.0	17.2
Operating Segments | Corporate | Commission income
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	78.7	78.5	154.5	157.5
Operating Segments | Corporate | Revenue by type - Eliminations
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	$ (47.4)	$ (50.7)	$ (95.0)	$ (93.8)