Schedule III - Supplementary Insurance Information (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Supplementary Insurance Information
Deferred acquisition costs	$ 2,993.0	$ 3,077.0	$ 2,590.0	$ 2,358.1
Future policy benefits and claims	24,036.6	22,626.2
Contractholder and other policyholder funds	35,539.4	36,717.2
Premiums and other considerations	3,722.9	3,154.1	3,219.4
Net investment income (loss)	3,257.9	3,138.4	3,254.9
Benefits, claims and settlement expenses	5,231.0	4,683.6	5,123.9
Amortization of deferred acquisition costs	367.2	187.1	94.9
Other operating expenses	3,207.1	3,105.8	2,838.6
Retirement and Investor Services
Supplementary Insurance Information
Deferred acquisition costs	797.4	848.9
Future policy benefits and claims	11,274.2	9,975.6
Contractholder and other policyholder funds	28,968.5	30,537.5
Premiums and other considerations	1,584.5	1,045.5	1,162.6
Net investment income (loss)	1,857.9	1,933.3	2,075.4
Benefits, claims and settlement expenses	2,766.9	2,303.4	2,629.1
Amortization of deferred acquisition costs	163.6	75.0	84.8
Other operating expenses	1,603.1	1,529.9	1,362.5
Principal Global Investors
Supplementary Insurance Information
Net investment income (loss)	10.3	17.0	15.4
Other operating expenses	534.1	545.7	456.9
Principal International
Supplementary Insurance Information
Deferred acquisition costs	238.1	228.2
Future policy benefits and claims	4,071.8	4,220.2
Contractholder and other policyholder funds	275.0	172.2
Premiums and other considerations	225.7	291.6	284.6
Net investment income (loss)	591.0	447.1	435.2
Benefits, claims and settlement expenses	622.9	569.0	567.2
Amortization of deferred acquisition costs	18.7	16.5	12.6
Other operating expenses	342.8	318.0	208.5
U.S. Insurance Solutions
Supplementary Insurance Information
Deferred acquisition costs	1,957.5	1,999.9
Future policy benefits and claims	8,652.6	8,389.0
Contractholder and other policyholder funds	6,602.3	6,293.9
Premiums and other considerations	1,913.8	1,816.5	1,769.3
Net investment income (loss)	714.8	686.3	675.2
Benefits, claims and settlement expenses	1,847.9	1,826.0	1,935.5
Amortization of deferred acquisition costs	184.9	95.6	(2.5)
Other operating expenses	714.7	689.0	662.0
Corporate
Supplementary Insurance Information
Future policy benefits and claims	38.0	41.4
Contractholder and other policyholder funds	(306.4)	(286.4)
Premiums and other considerations	(1.1)	0.5	2.9
Net investment income (loss)	83.9	54.7	53.7
Benefits, claims and settlement expenses	(6.7)	(14.8)	(7.9)
Other operating expenses	$ 12.4	$ 23.2	$ 148.7