Schedule III - Supplementary Insurance Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplementary Insurance Information
Deferred acquisition costs	$ 3,693.5	$ 3,540.7	$ 3,380.2	$ 3,276.1
Future policy benefits and claims	35,664.8	33,019.3
Contractholder and other policyholder funds	40,588.1	39,004.8
Premiums and other considerations	6,409.6	6,217.4	5,299.1
Net investment income	3,629.2	3,459.3	3,296.5
Benefits, claims and settlement expenses	8,192.5	7,822.6	6,913.2
Amortization of deferred acquisition costs	253.5	234.6	285.1
Other operating expenses	3,883.2	3,659.2	3,447.5
Retirement and Income Solutions
Supplementary Insurance Information
Deferred acquisition costs	988.3	887.5
Future policy benefits and claims	21,217.9	18,437.2
Contractholder and other policyholder funds	32,295.1	30,767.9
Premiums and other considerations	3,635.9	3,671.1	2,860.9
Net investment income	2,204.1	2,069.2	1,931.2
Benefits, claims and settlement expenses	5,099.0	4,972.6	4,108.3
Amortization of deferred acquisition costs	124.5	35.8	161.4
Other operating expenses	1,191.2	1,158.1	1,114.3
Principal Global Investors
Supplementary Insurance Information
Net investment income	4.2	13.4	12.6
Other operating expenses	1,176.9	974.7	935.8
Principal International
Supplementary Insurance Information
Deferred acquisition costs	13.1	209.0
Future policy benefits and claims	4,317.6	4,703.6
Contractholder and other policyholder funds	1,183.1	1,252.4
Premiums and other considerations	317.2	218.0	274.6
Net investment income	494.9	510.6	491.4
Benefits, claims and settlement expenses	646.6	507.1	584.6
Amortization of deferred acquisition costs	1.3	22.4	22.5
Other operating expenses	467.9	404.3	365.5
U.S. Insurance Solutions
Supplementary Insurance Information
Deferred acquisition costs	2,692.1	2,444.2
Future policy benefits and claims	9,950.3	9,700.1
Contractholder and other policyholder funds	7,458.4	7,322.3
Premiums and other considerations	2,456.5	2,328.3	2,163.6
Net investment income	806.2	775.0	751.2
Benefits, claims and settlement expenses	2,444.6	2,345.3	2,221.4
Amortization of deferred acquisition costs	127.7	176.4	101.2
Other operating expenses	939.9	867.7	796.4
Corporate
Supplementary Insurance Information
Future policy benefits and claims	179.0	178.4
Contractholder and other policyholder funds	(348.5)	(337.8)
Net investment income	119.8	91.1	110.1
Benefits, claims and settlement expenses	2.3	(2.4)	(1.1)
Other operating expenses	$ 107.3	$ 254.4	$ 235.5