Revenues from Contracts with Customers - Disaggregation within Segments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenues from Contracts with Customers - Disaggregation of Revenues
Total fees and other revenues	$ 1,012.6	$ 955.1	$ 2,023.9	$ 1,895.7
Premiums and other considerations	1,265.8	1,452.7	2,261.0	2,700.7
Net investment income (loss)	889.1	866.4	1,791.3	1,743.8
Operating Segments
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	614.3	567.8	1,217.1	1,119.1
Total operating revenues	3,206.2	3,309.5	6,153.4	6,414.2
Operating Segments | Retirement and Income Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	69.6	68.1	134.0	133.2
Total operating revenues	1,450.8	1,683.8	2,632.9	3,166.4
Operating Segments | Principal Global Investors
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	371.1	344.0	731.4	684.3
Fees and other revenues not within the scope of revenue recognition guidance	7.3	10.2	14.0	14.6
Total fees and other revenues	378.4	354.2	745.4	698.9
Net investment income (loss)	1.2	1.3	2.7	2.5
Total operating revenues	379.6	355.5	748.1	701.4
Operating Segments | Principal International
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	123.1	105.7	244.2	206.5
Fees and other revenues not within the scope of revenue recognition guidance	1.8	1.6	3.8	3.4
Total fees and other revenues	124.9	107.3	248.0	209.9
Premiums and other considerations	101.6	55.4	199.5	107.4
Net investment income (loss)	163.1	156.7	325.2	323.4
Total operating revenues	389.6	319.4	772.7	640.7
Operating Segments | Principal International | Latin America
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	96.5	78.6	189.0	154.9
Operating Segments | Principal International | Asia
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	26.7	27.2	55.4	51.9
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	0.6	0.4
Operating Segments | Principal International | Geographical Eliminations
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	(0.4)	(0.3)	(0.8)	(0.7)
Operating Segments | U.S. Insurance Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	13.5	12.5	26.6	25.5
Total operating revenues	1,003.2	970.6	2,016.7	1,927.0
Operating Segments | Corporate
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	37.0	37.5	80.9	69.6
Fees and other revenues not within the scope of revenue recognition guidance	(78.7)	(74.3)	(157.3)	(135.2)
Total fees and other revenues	(41.7)	(36.8)	(76.4)	(65.6)
Net investment income (loss)	24.7	17.0	59.4	44.3
Total operating revenues	(17.0)	(19.8)	(17.0)	(21.3)
Operating Segments | Management fee revenue | Principal Global Investors
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	312.2	300.3	628.5	592.2
Operating Segments | Management fee revenue | Principal International
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	121.8	101.6	241.9	199.9
Operating Segments | Other fee revenue | Principal Global Investors
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	58.9	43.7	102.9	92.1
Operating Segments | Other fee revenue | Principal International
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	1.3	4.1	2.3	6.6
Operating Segments | Other fee revenue | Corporate
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	9.2	8.1	17.2	14.5
Operating Segments | Commission income | Corporate
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	78.5	78.2	157.5	151.3
Operating Segments | Revenue by type - Eliminations | Corporate
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	(50.7)	(48.8)	(93.8)	(96.2)
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	66.9	65.6	128.6	128.2
Fees and other revenues not within the scope of revenue recognition guidance	282.8	271.4	569.2	538.5
Total fees and other revenues	349.7	337.0	697.8	666.7
Premiums and other considerations	0.9	0.7	2.4	1.8
Net investment income (loss)	106.2	113.8	216.2	233.4
Total operating revenues	456.8	451.5	916.4	901.9
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	66.3	65.1	127.9	127.6
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	0.6	0.5	0.7	0.6
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	5.4	5.0
Fees and other revenues not within the scope of revenue recognition guidance	2.9	4.2	5.6	8.0
Total fees and other revenues	5.6	6.7	11.0	13.0
Premiums and other considerations	555.1	814.0	843.4	1,437.6
Net investment income (loss)	433.3	411.6	862.1	813.9
Total operating revenues	994.0	1,232.3	1,716.5	2,264.5
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	5.4	5.0
Specialty Benefits insurance | Operating Segments | U.S. Insurance Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	3.8	3.6	7.5	6.9
Fees and other revenues not within the scope of revenue recognition guidance	5.2	5.4	10.4	10.8
Total fees and other revenues	9.0	9.0	17.9	17.7
Premiums and other considerations	527.7	490.2	1,044.7	969.1
Net investment income (loss)	35.8	38.1	76.3	79.0
Total operating revenues	572.5	537.3	1,138.9	1,065.8
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	3.8	3.6	7.5	6.9
Individual Life insurance | Operating Segments | U.S. Insurance Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total revenues from contracts with customers	9.8	9.0	19.2	18.7
Fees and other revenues not within the scope of revenue recognition guidance	177.7	168.8	363.4	336.7
Total fees and other revenues	187.5	177.8	382.6	355.4
Premiums and other considerations	80.5	92.4	171.0	184.8
Net investment income (loss)	162.8	163.2	324.3	321.1
Total operating revenues	430.8	433.4	877.9	861.3
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	5.4	11.6	10.6
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	$ 4.0	$ 3.6	$ 7.6	$ 8.1