Schedule III - Supplementary Insurance Information (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Supplementary Insurance Information
Deferred policy acquisition costs	$ 3,313.5	$ 3,529.8	$ 3,681.4	$ 4,153.0
Future policy benefits and claims	20,207.9	20,046.3
Contractholder and other policyholder funds	38,220.1	37,893.3
Premiums and other considerations	2,891.0	3,555.5	3,750.6
Net investment income (loss)	3,375.8	3,496.5	3,400.8
Benefits, claims and settlement expenses	4,454.1	5,338.4	5,334.5
Amortization of deferred policy acquisition costs	538.0	205.9	92.2
Other operating expenses	2,519.7	2,553.1	2,434.4
Retirement and Investor Services
Supplementary Insurance Information
Deferred policy acquisition costs	1,276.0	1,321.1
Future policy benefits and claims	8,563.1	8,299.5
Contractholder and other policyholder funds	33,268.9	33,448.4
Premiums and other considerations	390.4	332.2	247.2
Net investment income (loss)	2,182.8	2,366.5	2,491.2
Benefits, claims and settlement expenses	1,933.3	2,122.8	2,185.1
Amortization of deferred policy acquisition costs	222.1	192.2	54.0
Other operating expenses	1,136.3	1,059.1	1,019.5
Principal Global Investors
Supplementary Insurance Information
Net investment income (loss)	15.1	13.4	9.6
Other operating expenses	429.3	384.4	376.1
Principal International
Supplementary Insurance Information
Deferred policy acquisition costs	279.0	271.0
Future policy benefits and claims	3,821.3	3,969.3
Contractholder and other policyholder funds	83.1	68.8
Premiums and other considerations	264.5	255.2	239.1
Net investment income (loss)	470.4	375.5	209.0
Benefits, claims and settlement expenses	581.7	497.7	328.5
Amortization of deferred policy acquisition costs	(3.7)	4.3	(1.7)
Other operating expenses	170.6	139.5	115.8
U.S. Insurance Solutions
Supplementary Insurance Information
Deferred policy acquisition costs	1,758.5	1,937.7
Future policy benefits and claims	7,754.8	7,586.4
Contractholder and other policyholder funds	5,128.1	4,599.8
Premiums and other considerations	1,724.0	1,685.7	1,776.6
Net investment income (loss)	669.9	650.0	620.5
Benefits, claims and settlement expenses	1,562.6	1,734.3	1,644.9
Amortization of deferred policy acquisition costs	319.6	9.4	39.9
Other operating expenses	586.6	541.8	545.7
Corporate
Supplementary Insurance Information
Future policy benefits and claims	68.7	191.1
Contractholder and other policyholder funds	(260.0)	(223.7)
Premiums and other considerations	512.1	1,282.4	1,487.7
Net investment income (loss)	37.6	91.1	70.5
Benefits, claims and settlement expenses	376.5	983.6	1,176.0
Other operating expenses	$ 196.9	$ 428.3	$ 377.3