Revenues from Contracts with Customers - Disaggregation within Segments (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenues from Contracts with Customers - Disaggregation of Revenues
Total fees and other revenues		$ 4,409.9	$ 4,273.8	$ 3,892.3
Premiums and other considerations		7,866.6	6,409.6	6,217.4
Net investment income (loss)		3,998.4	3,629.2	3,459.3
Retirement and Income Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Premiums and other considerations		4,862.7	3,635.9	3,671.1
Net investment income (loss)		2,420.4	2,195.2	2,058.3
Principal Global Investors
Revenues from Contracts with Customers - Disaggregation of Revenues
Net investment income (loss)		10.1	4.2	13.4
Principal International
Revenues from Contracts with Customers - Disaggregation of Revenues
Premiums and other considerations		393.3	317.2	218.0
Net investment income (loss)		575.9	494.9	510.6
U.S. Insurance Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Premiums and other considerations		2,610.6	2,456.5	2,328.3
Net investment income (loss)		854.5	815.1	785.9
Corporate
Revenues from Contracts with Customers - Disaggregation of Revenues
Net investment income (loss)		137.5	119.8	91.1
Operating Segments
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers		2,610.0	2,658.9	2,299.2
Total operating revenues		16,393.1	14,424.8	13,703.3
Operating Segments | Retirement and Income Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers		464.8	262.9	268.2
Total operating revenues		8,954.4	7,277.0	7,154.7
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		455.5	252.4	258.0
Fees and other revenues not within the scope of revenue recognition guidance		1,139.0	1,132.9	1,111.2
Total fees and other revenues		1,594.5	1,385.3	1,369.2
Premiums and other considerations		3.5	4.0	3.8
Net investment income (loss)		405.0	394.4	403.4
Total operating revenues		2,003.0	1,783.7	1,776.4
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		453.7	250.7	256.4
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		1.8	1.7	1.6
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		9.3	10.5	10.2
Fees and other revenues not within the scope of revenue recognition guidance		13.9	12.5	15.0
Total fees and other revenues		23.2	23.0	25.2
Premiums and other considerations		4,859.2	3,631.9	3,667.3
Net investment income (loss)		2,069.0	1,838.4	1,685.8
Total operating revenues		6,951.4	5,493.3	5,378.3
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		9.3	10.5	10.2
Operating Segments | Principal Global Investors
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers		1,456.7	1,700.5	1,406.1
Fees and other revenues not within the scope of revenue recognition guidance		38.5	30.4	28.1
Total fees and other revenues		1,495.2	1,730.9	1,434.2
Net investment income (loss)		10.6	5.4	10.2
Total operating revenues		1,505.8	1,736.3	1,444.4
Operating Segments | Principal Global Investors | Management fee revenue
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers		1,239.1	1,244.5	1,217.0
Operating Segments | Principal Global Investors | Other fee revenue
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers		217.6	456.0	189.1
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		462.2	477.1	426.4
Fees and other revenues not within the scope of revenue recognition guidance		6.3	7.3	7.1
Total fees and other revenues		468.5	484.4	433.5
Premiums and other considerations		393.3	317.2	218.0
Net investment income (loss)		661.4	571.3	600.0
Total operating revenues		1,523.2	1,372.9	1,251.5
Operating Segments | Principal International | Latin America
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers		351.5	370.8	318.9
Operating Segments | Principal International | Asia
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers		111.0	106.3	108.3
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	0.7
Operating Segments | Principal International | Geographical Eliminations
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers		(1.2)	(0.9)	(1.5)
Operating Segments | Principal International | Management fee revenue
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers		459.3	472.1	414.6
Operating Segments | Principal International | Other fee revenue
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers		2.9	5.0	11.8
Operating Segments | U.S. Insurance Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers		65.9	57.0	50.1
Total operating revenues		4,449.0	4,089.8	3,913.5
Operating Segments | U.S. Insurance Solutions | Specialty Benefits insurance
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers		14.9	14.8	13.9
Fees and other revenues not within the scope of revenue recognition guidance		19.6	20.6	21.3
Total fees and other revenues		34.5	35.4	35.2
Premiums and other considerations		2,292.7	2,134.1	1,985.9
Net investment income (loss)		166.4	156.9	147.3
Total operating revenues		2,493.6	2,326.4	2,168.4
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		14.9	14.8	13.9
Operating Segments | U.S. Insurance Solutions | Individual Life insurance
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers		51.2	42.4	36.4
Fees and other revenues not within the scope of revenue recognition guidance		875.9	727.0	703.5
Total fees and other revenues		927.1	769.4	739.9
Premiums and other considerations		317.9	322.4	342.4
Net investment income (loss)		710.6	671.8	663.0
Total operating revenues		1,955.6	1,763.6	1,745.3
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		24.3	22.8	21.5
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		26.9	19.6	14.9
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		(0.2)	(0.2)	(0.2)
Total operating revenues		(0.2)	(0.2)	(0.2)
Operating Segments | Corporate
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers		160.4	161.4	148.4
Fees and other revenues not within the scope of revenue recognition guidance		(301.8)	(316.3)	(292.8)
Total fees and other revenues		(141.4)	(154.9)	(144.4)
Net investment income (loss)		102.1	103.7	83.6
Total operating revenues		(39.3)	(51.2)	(60.8)
Operating Segments | Corporate | Other fee revenue
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers		46.1	35.4	30.1
Operating Segments | Corporate | Commission income
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers		320.2	316.1	299.7
Operating Segments | Corporate | Revenue by type - Eliminations
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers		$ (205.9)	$ (190.1)	$ (181.4)