Schedule III - Supplementary Insurance Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Supplementary Insurance Information
Deferred acquisition costs	$ 3,521.3	$ 3,693.5	$ 3,540.7	$ 3,380.2
Future policy benefits and claims	40,838.2	35,664.8
Contractholder and other policyholder funds	42,326.9	40,588.1
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
Benefits, claims and settlement expenses	9,905.8	8,192.5	7,822.6
Amortization of deferred acquisition costs	347.0	253.5	234.6
Other operating expenses	4,156.9	3,883.2	3,659.2
Retirement and Income Solutions
Supplementary Insurance Information
Deferred acquisition costs	833.9	988.3
Future policy benefits and claims	25,305.1	21,217.9
Contractholder and other policyholder funds	33,814.4	32,295.1
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
Benefits, claims and settlement expenses	6,527.8	5,099.0	4,972.6
Amortization of deferred acquisition costs	87.8	124.5	35.8
Other operating expenses	1,403.5	1,140.7	1,108.5
Principal Global Investors
Supplementary Insurance Information
Net investment income (loss)	10.1	4.2	13.4
Other operating expenses	1,028.5	1,168.0	963.1
Principal International
Supplementary Insurance Information
Deferred acquisition costs	11.3	13.1
Future policy benefits and claims	4,537.1	4,317.6
Contractholder and other policyholder funds	1,145.0	1,183.1
Premiums and other considerations	393.3	317.2	218.0
Net investment income (loss)	575.9	494.9	510.6
Benefits, claims and settlement expenses	728.3	646.6	507.1
Amortization of deferred acquisition costs	1.3	1.3	22.4
Other operating expenses	436.0	451.0	388.1
U.S. Insurance Solutions
Supplementary Insurance Information
Deferred acquisition costs	2,676.1	2,692.1
Future policy benefits and claims	10,823.5	9,950.3
Contractholder and other policyholder funds	7,720.0	7,458.4
Premiums and other considerations	2,610.6	2,456.5	2,328.3
Net investment income (loss)	854.5	815.1	785.9
Benefits, claims and settlement expenses	2,639.2	2,444.6	2,345.3
Amortization of deferred acquisition costs	257.9	127.7	176.4
Other operating expenses	946.5	925.4	850.5
Corporate
Supplementary Insurance Information
Future policy benefits and claims	172.5	179.0
Contractholder and other policyholder funds	(352.5)	(348.5)
Net investment income (loss)	137.5	119.8	91.1
Benefits, claims and settlement expenses	10.5	2.3	(2.4)
Other operating expenses	$ 342.4	$ 198.1	$ 349.0