Schedule III - Supplementary Insurance Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Supplementary Insurance Information
Deferred acquisition costs	$ 3,540.7	$ 3,380.2	$ 3,276.1	$ 2,993.0
Future policy benefits and claims	33,019.3	29,000.7
Contractholder and other policyholder funds	39,004.8	38,844.0
Premiums and other considerations	6,217.4	5,299.1	5,310.3
Net investment income	3,459.3	3,296.5	3,052.1
Benefits, claims and settlement expenses	7,822.6	6,913.2	6,697.7
Amortization of deferred acquisition costs	234.6	285.1	270.8
Other operating expenses	3,659.2	3,447.5	3,401.6
Retirement and Income Solutions
Supplementary Insurance Information
Deferred acquisition costs	887.5	812.7
Future policy benefits and claims	18,437.2	15,455.0
Contractholder and other policyholder funds	30,767.9	30,796.8
Premiums and other considerations	3,671.1	2,860.9	3,011.4
Net investment income	2,069.2	1,931.2	1,749.0
Benefits, claims and settlement expenses	4,972.6	4,108.3	4,158.4
Amortization of deferred acquisition costs	35.8	161.4	158.2
Other operating expenses	1,158.1	1,114.3	1,126.8
Principal Global Investors
Supplementary Insurance Information
Net investment income	13.4	12.6	8.1
Other operating expenses	974.7	935.8	943.4
Principal International
Supplementary Insurance Information
Deferred acquisition costs	209.0	196.0
Future policy benefits and claims	4,703.6	4,182.2
Contractholder and other policyholder funds	1,252.4	1,316.9
Premiums and other considerations	218.0	274.6	252.9
Net investment income	510.6	491.4	495.9
Benefits, claims and settlement expenses	507.1	584.6	573.9
Amortization of deferred acquisition costs	22.4	22.5	19.1
Other operating expenses	404.3	365.5	368.9
U.S. Insurance Solutions
Supplementary Insurance Information
Deferred acquisition costs	2,444.2	2,371.5
Future policy benefits and claims	9,700.1	9,171.5
Contractholder and other policyholder funds	7,322.3	7,060.7
Premiums and other considerations	2,328.3	2,163.6	2,045.9
Net investment income	775.0	751.2	717.8
Benefits, claims and settlement expenses	2,345.3	2,221.4	1,963.5
Amortization of deferred acquisition costs	176.4	101.2	93.5
Other operating expenses	867.7	796.4	788.1
Corporate
Supplementary Insurance Information
Future policy benefits and claims	178.4	192.0
Contractholder and other policyholder funds	(337.8)	(330.4)
Premiums and other considerations			0.1
Net investment income	91.1	110.1	81.3
Benefits, claims and settlement expenses	(2.4)	(1.1)	1.9
Other operating expenses	$ 254.4	$ 235.5	$ 174.4