Revenues from Contracts with Customers - Disaggregation within Segments (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenues from Contracts with Customers - Disaggregation of Revenues
Total fees and other revenues		$ 4,511.1	$ 4,409.9	$ 4,273.8
Premiums and other considerations		6,037.4	7,866.6	6,409.6
Net investment income (loss)		3,890.6	3,998.4	3,629.2
Retirement and Income Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Premiums and other considerations		3,221.0	4,862.7	3,635.9
Net investment income (loss)		2,457.9	2,420.4	2,195.2
Principal Global Investors
Revenues from Contracts with Customers - Disaggregation of Revenues
Net investment income (loss)		5.6	10.1	4.2
Principal International
Revenues from Contracts with Customers - Disaggregation of Revenues
Premiums and other considerations		156.6	393.3	317.2
Net investment income (loss)		446.8	575.9	494.9
U.S. Insurance Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Premiums and other considerations		2,659.8	2,610.6	2,456.5
Net investment income (loss)		850.6	854.5	815.1
Corporate
Revenues from Contracts with Customers - Disaggregation of Revenues
Net investment income (loss)		129.7	137.5	119.8
Operating Segments
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers		2,768.3	2,610.0	2,658.9
Total operating revenues		14,579.9	16,393.1	14,424.8
Operating Segments | Retirement and Income Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers		599.8	464.8	262.9
Total operating revenues		7,503.5	8,954.4	7,277.0
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		591.2	455.5	252.4
Fees and other revenues not within the scope of revenue recognition guidance		1,130.4	1,139.0	1,132.9
Total fees and other revenues		1,721.6	1,594.5	1,385.3
Premiums and other considerations		5.0	3.5	4.0
Net investment income (loss)		423.2	405.0	394.4
Total operating revenues		2,149.8	2,003.0	1,783.7
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		589.2	453.7	250.7
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		2.0	1.8	1.7
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		8.6	9.3	10.5
Fees and other revenues not within the scope of revenue recognition guidance		9.5	13.9	12.5
Total fees and other revenues		18.1	23.2	23.0
Premiums and other considerations		3,216.0	4,859.2	3,631.9
Net investment income (loss)		2,119.6	2,069.0	1,838.4
Total operating revenues		5,353.7	6,951.4	5,493.3
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		8.4	9.3	10.5
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
Operating Segments | Principal Global Investors
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers		1,511.2	1,456.7	1,700.5
Fees and other revenues not within the scope of revenue recognition guidance		22.3	38.5	30.4
Total fees and other revenues		1,533.5	1,495.2	1,730.9
Net investment income (loss)		5.6	10.6	5.4
Total operating revenues		1,539.1	1,505.8	1,736.3
Operating Segments | Principal Global Investors | Management fee revenue
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers		1,298.4	1,239.1	1,244.5
Operating Segments | Principal Global Investors | Other fee revenue
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers		212.8	217.6	456.0
Performance fee recognized resulting from realignment of a real estate investment team	$ 253.1
Operating Segments | Principal International
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers		439.2	462.2	477.1
Fees and other revenues not within the scope of revenue recognition guidance		5.6	6.3	7.3
Total fees and other revenues		444.8	468.5	484.4
Premiums and other considerations		156.6	393.3	317.2
Net investment income (loss)		495.4	661.4	571.3
Total operating revenues		1,096.8	1,523.2	1,372.9
Operating Segments | Principal International | Latin America
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers		323.5	351.5	370.8
Operating Segments | Principal International | Asia
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers		115.9	111.0	106.3
Operating Segments | Principal International | Principal International Corporate / Regional Office
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers		0.9	0.9	0.9
Operating Segments | Principal International | Geographical Eliminations
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers		(1.1)	(1.2)	(0.9)
Operating Segments | Principal International | Management fee revenue
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers		435.3	459.3	472.1
Operating Segments | Principal International | Other fee revenue
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers		3.9	2.9	5.0
Operating Segments | U.S. Insurance Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers		63.2	65.9	57.0
Total operating revenues		4,480.2	4,449.0	4,089.8
Operating Segments | U.S. Insurance Solutions | Specialty Benefits insurance
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers		14.8	14.9	14.8
Fees and other revenues not within the scope of revenue recognition guidance		19.3	19.6	20.6
Total fees and other revenues		34.1	34.5	35.4
Premiums and other considerations		2,330.7	2,292.7	2,134.1
Net investment income (loss)		160.6	166.4	156.9
Total operating revenues		2,525.4	2,493.6	2,326.4
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		14.8	14.9	14.8
Operating Segments | U.S. Insurance Solutions | Individual Life insurance
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers		48.6	51.2	42.4
Fees and other revenues not within the scope of revenue recognition guidance		849.8	875.9	727.0
Total fees and other revenues		898.4	927.1	769.4
Premiums and other considerations		329.1	317.9	322.4
Net investment income (loss)		727.5	710.6	671.8
Total operating revenues		1,955.0	1,955.6	1,763.6
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		21.8	24.3	22.8
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		26.8	26.9	19.6
Operating Segments | Corporate
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers		154.9	160.4	161.4
Fees and other revenues not within the scope of revenue recognition guidance		(303.9)	(301.8)	(316.3)
Total fees and other revenues		(149.0)	(141.4)	(154.9)
Net investment income (loss)		109.3	102.1	103.7
Total operating revenues		(39.7)	(39.3)	(51.2)
Operating Segments | Corporate | Other fee revenue
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers		59.1	46.1	35.4
Operating Segments | Corporate | Commission income
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers		316.6	320.2	316.1
Operating Segments | Corporate | Revenue by type - Eliminations
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers		$ (220.8)	$ (205.9)	$ (190.1)