Schedule III - Supplementary Insurance Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Supplementary Insurance Information
Deferred acquisition costs	$ 3,276.1	$ 2,993.0	$ 3,077.0	$ 2,590.0
Future policy benefits and claims	25,856.5	24,036.6
Contractholder and other policyholder funds	36,521.5	35,539.4
Premiums and other considerations	5,310.3	3,722.9	3,154.1
Net investment income (loss)	3,052.1	3,257.9	3,138.4
Benefits, claims and settlement expenses	6,697.7	5,231.0	4,683.6
Amortization of deferred acquisition costs	270.8	367.2	187.1
Other operating expenses	3,401.6	3,207.1	3,105.8
Retirement and Income Solutions
Supplementary Insurance Information
Deferred acquisition costs	854.0	797.4
Future policy benefits and claims	13,229.8	11,274.2
Contractholder and other policyholder funds	28,651.5	28,968.5
Premiums and other considerations	3,011.4	1,584.5	1,045.5
Net investment income (loss)	1,749.0	1,857.7	1,933.1
Benefits, claims and settlement expenses	4,158.4	2,766.9	2,303.4
Amortization of deferred acquisition costs	158.2	163.6	75.0
Other operating expenses	1,126.8	1,056.9	1,032.3
Principal Global Investors
Supplementary Insurance Information
Net investment income (loss)	8.1	10.5	17.2
Other operating expenses	943.4	895.3	884.7
Principal International
Supplementary Insurance Information
Deferred acquisition costs	218.8	238.1
Future policy benefits and claims	3,634.9	4,071.8
Contractholder and other policyholder funds	1,403.9	275.0
Premiums and other considerations	252.9	225.7	291.6
Net investment income (loss)	495.9	591.0	447.1
Benefits, claims and settlement expenses	573.9	622.9	569.0
Amortization of deferred acquisition costs	19.1	18.7	16.5
Other operating expenses	368.9	342.8	318.0
U.S. Insurance Solutions
Supplementary Insurance Information
Deferred acquisition costs	2,203.3	1,957.5
Future policy benefits and claims	8,953.2	8,652.6
Contractholder and other policyholder funds	6,784.2	6,602.3
Premiums and other considerations	2,045.9	1,913.8	1,816.5
Net investment income (loss)	717.8	714.8	686.3
Benefits, claims and settlement expenses	1,963.5	1,847.9	1,826.0
Amortization of deferred acquisition costs	93.5	184.9	95.6
Other operating expenses	788.1	714.7	689.0
Corporate
Supplementary Insurance Information
Future policy benefits and claims	38.6	38.0
Contractholder and other policyholder funds	(318.1)	(306.4)
Premiums and other considerations	0.1	(1.1)	0.5
Net investment income (loss)	81.3	83.9	54.7
Benefits, claims and settlement expenses	1.9	(6.7)	(14.8)
Other operating expenses	$ 174.4	$ 197.4	$ 181.8