Revenues from Contracts with Customers - Disaggregation within Segments (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenues from Contracts with Customers - Disaggregation of Revenues
Total fees and other revenues	$ 1,011.3	$ 940.6
Premiums and other considerations	995.2	1,248.0
Net investment income	902.2	877.4
Operating Segments
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	602.8	551.3
Total operating revenues	2,947.2	3,104.7
Operating Segments | Retirement and Income Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	64.4	65.1
Total operating revenues	1,182.1	1,482.6
Operating Segments | Principal Global Investors
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	360.3	340.3
Fees and other revenues not within the scope of revenue recognition guidance	6.7	4.4
Total fees and other revenues	367.0	344.7
Net investment income	1.5	1.2
Total operating revenues	368.5	345.9
Operating Segments | Principal International
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	121.1	100.8
Fees and other revenues not within the scope of revenue recognition guidance	2.0	1.8
Total fees and other revenues	123.1	102.6
Premiums and other considerations	97.9	52.0
Net investment income	162.1	166.7
Total operating revenues	383.1	321.3
Operating Segments | Principal International | Latin America
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	92.5	76.3
Operating Segments | Principal International | Asia
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	28.7	24.7
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
Operating Segments | Principal International | Geographical Eliminations
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	(0.4)	(0.4)
Operating Segments | U.S. Insurance Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	13.1	13.0
Total operating revenues	1,013.5	956.4
Operating Segments | Corporate
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	43.9	32.1
Fees and other revenues not within the scope of revenue recognition guidance	(78.6)	(60.9)
Total fees and other revenues	(34.7)	(28.8)
Net investment income	34.7	27.3
Total operating revenues		(1.5)
Operating Segments | Management fee revenue | Principal Global Investors
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	316.3	291.9
Operating Segments | Management fee revenue | Principal International
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	120.1	98.3
Operating Segments | Other fee revenue | Principal Global Investors
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	44.0	48.4
Operating Segments | Other fee revenue | Principal International
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	1.0	2.5
Operating Segments | Other fee revenue | Corporate
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	8.0	6.4
Operating Segments | Commission income | Corporate
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	79.0	73.1
Operating Segments | Revenue by type - Eliminations | Corporate
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	(43.1)	(47.4)
Retirement and Income Solutions - Fee | Operating Segments | Retirement and Income Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	61.7	62.6
Fees and other revenues not within the scope of revenue recognition guidance	286.4	267.1
Total fees and other revenues	348.1	329.7
Premiums and other considerations	1.5	1.1
Net investment income	110.0	119.6
Total operating revenues	459.6	450.4
Retirement and Income Solutions - Fee | Operating Segments | Administrative service fee revenue | Retirement and Income Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	61.6	62.5
Retirement and Income Solutions - Fee | Operating Segments | Other fee revenue | Retirement and Income Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	0.1	0.1
Retirement and Income Solutions - Spread | Operating Segments | Retirement and Income Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	2.7	2.5
Fees and other revenues not within the scope of revenue recognition guidance	2.7	3.8
Total fees and other revenues	5.4	6.3
Premiums and other considerations	288.3	623.6
Net investment income	428.8	402.3
Total operating revenues	722.5	1,032.2
Retirement and Income Solutions - Spread | Operating Segments | Deposit account fee revenue | Retirement and Income Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	2.7	2.5
Specialty benefits insurance | Operating Segments | U.S. Insurance Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	3.7	3.3
Fees and other revenues not within the scope of revenue recognition guidance	5.2	5.4
Total fees and other revenues	8.9	8.7
Premiums and other considerations	517.0	478.9
Net investment income	40.5	40.9
Total operating revenues	566.4	528.5
Specialty benefits insurance | Operating Segments | Administrative service fee revenue | U.S. Insurance Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	3.7	3.3
Individual life insurance | Operating Segments | U.S. Insurance Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	9.4	9.7
Fees and other revenues not within the scope of revenue recognition guidance	185.7	167.9
Total fees and other revenues	195.1	177.6
Premiums and other considerations	90.5	92.4
Net investment income	161.5	157.9
Total operating revenues	447.1	427.9
Individual life insurance | Operating Segments | Administrative service fee revenue | U.S. Insurance Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	5.8	5.2
Individual life insurance | Operating Segments | Commission income | U.S. Insurance Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	$ 3.6	$ 4.5