Schedule III - Supplementary Insurance Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Supplementary Insurance Information
Deferred acquisition costs	$ 3,409.7	$ 3,521.3	$ 3,693.5	$ 3,540.7
Future policy benefits and claims	45,207.2	40,838.2
Contractholder and other policyholder funds	44,297.1	42,326.9
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
Benefits, claims and settlement expenses	8,281.5	9,905.8	8,192.5
Amortization of deferred acquisition costs	388.1	347.0	253.5
Other operating expenses	4,258.4	4,156.9	3,883.2
Retirement and Income Solutions
Supplementary Insurance Information
Deferred acquisition costs	730.4	833.9
Future policy benefits and claims	28,134.1	25,305.1
Contractholder and other policyholder funds	35,576.0	33,814.4
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
Benefits, claims and settlement expenses	4,899.4	6,527.8	5,099.0
Amortization of deferred acquisition costs	82.2	87.8	124.5
Other operating expenses	1,578.2	1,403.5	1,140.7
Principal Global Investors
Supplementary Insurance Information
Net investment income (loss)	5.6	10.1	4.2
Other operating expenses	1,029.6	1,028.5	1,168.0
Principal International
Supplementary Insurance Information
Deferred acquisition costs	11.2	11.3
Future policy benefits and claims	5,109.5	4,537.1
Contractholder and other policyholder funds	1,167.3	1,145.0
Premiums and other considerations	156.6	393.3	317.2
Net investment income (loss)	446.8	575.9	494.9
Benefits, claims and settlement expenses	440.7	728.3	646.6
Amortization of deferred acquisition costs	1.2	1.3	1.3
Other operating expenses	416.8	436.0	451.0
U.S. Insurance Solutions
Supplementary Insurance Information
Deferred acquisition costs	2,668.1	2,676.1
Future policy benefits and claims	11,800.3	10,823.5
Contractholder and other policyholder funds	7,909.5	7,720.0
Premiums and other considerations	2,659.8	2,610.6	2,456.5
Net investment income (loss)	850.6	854.5	815.1
Benefits, claims and settlement expenses	2,937.2	2,639.2	2,444.6
Amortization of deferred acquisition costs	304.7	257.9	127.7
Other operating expenses	936.9	946.5	925.4
Corporate
Supplementary Insurance Information
Future policy benefits and claims	163.3	172.5
Contractholder and other policyholder funds	(355.7)	(352.5)
Net investment income (loss)	129.7	137.5	119.8
Benefits, claims and settlement expenses	4.2	10.5	2.3
Other operating expenses	$ 296.9	$ 342.4	$ 198.1