Schedule II - Condensed Financial Information of Registrant (Parent Only) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Assets
Fixed maturities, available-for-sale	$ 49,006.7	$ 48,636.3
Fixed maturities, trading	971.7	1,120.3
Cash and cash equivalents	2,833.9	1,877.4	2,240.4	2,608.0
Other investments	2,158.5	1,905.6
Net deferred income tax assets	15.5	13.0
Investment in unconsolidated entities	829.5	736.0
Total assets	148,298.0	145,631.1
Liabilities
Long-term debt	1,564.8	1,583.7
Total liabilities	138,285.0	135,746.1
Stockholders' equity
Common stock, par value $.01 per share - 2,500.0 million shares authorized, 450.3 million and 448.5 million shares issued, and 301.1 million and 320.4 million shares outstanding in 2011 and 2010	4.5	4.5
Additional paid-in capital	9,634.7	9,563.8
Retained earnings (deficit)	5,077.5	4,612.3
Accumulated other comprehensive income (loss)	201.9	272.4
Treasury stock, at cost (149.2 million and 128.1 million shares in 2011 and 2010)	(5,281.7)	(4,725.3)
Total stockholders' equity attributable to Principal Financial Group, Inc.	9,637.0	9,727.8
Total liabilities and stockholders' equity	148,298.0	145,631.1
Preferred stock, authorized (in shares)			13.0
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, authorized (in shares)	2,500.0	2,500.0
Common stock, issued (in shares)	450.3	448.5
Common stock, outstanding (in shares)	301.1	320.4
Treasury stock (in shares)	149.2	128.1
Series A
Stockholders' equity
Preferred stock, value
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, liquidation preference (in dollars per share)	$ 100	$ 100
Preferred stock, authorized (in shares)	3.0	3.0
Preferred stock, issued (in shares)	3.0	3.0
Preferred stock, outstanding (in shares)	3.0	3.0
Series B
Stockholders' equity
Preferred stock, value	0.1	0.1
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, liquidation preference (in dollars per share)	$ 25	$ 25
Preferred stock, authorized (in shares)	10.0	10.0
Preferred stock, issued (in shares)	10.0	10.0
Preferred stock, outstanding (in shares)	10.0	10.0
Principal Financial Group, Inc. Parent Only
Assets
Fixed maturities, available-for-sale		199.9
Fixed maturities, trading	268.7	274.9
Cash and cash equivalents	226.7	370.9	304.6	(2.0)
Other investments	7.0	5.6
Income taxes receivable	18.6	18.8
Net deferred income tax assets	22.5	21.6
Amounts receivable from subsidiary	0.9	0.8
Other assets	15.7	13.6
Investment in unconsolidated entities	10,447.1	10,195.1
Total assets	11,007.2	11,101.2
Liabilities
Amounts payable to subsidiary	1.4	5.5
Long-term debt	1,351.7	1,351.7
Accrued interest payable	15.4	16.1
Other liabilities	1.7	0.1
Total liabilities	1,370.2	1,373.4
Stockholders' equity
Common stock, par value $.01 per share - 2,500.0 million shares authorized, 450.3 million and 448.5 million shares issued, and 301.1 million and 320.4 million shares outstanding in 2011 and 2010	4.5	4.5
Additional paid-in capital	9,634.7	9,563.8
Retained earnings (deficit)	5,077.5	4,612.3
Accumulated other comprehensive income (loss)	201.9	272.4
Treasury stock, at cost (149.2 million and 128.1 million shares in 2011 and 2010)	(5,281.7)	(4,725.3)
Total stockholders' equity attributable to Principal Financial Group, Inc.	9,637.0	9,727.8
Total liabilities and stockholders' equity	11,007.2	11,101.2
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, authorized (in shares)	2,500.0	2,500.0
Common stock, issued (in shares)	450.3	448.5
Common stock, outstanding (in shares)	301.1	320.4
Treasury stock (in shares)	149.2	128.1
Principal Financial Group, Inc. Parent Only | Series A
Stockholders' equity
Preferred stock, value
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, liquidation preference (in dollars per share)	$ 100	$ 100
Preferred stock, authorized (in shares)	3.0	3.0
Preferred stock, issued (in shares)	3.0	3.0
Preferred stock, outstanding (in shares)	3.0	3.0
Principal Financial Group, Inc. Parent Only | Series B
Stockholders' equity
Preferred stock, value	$ 0.1	$ 0.1
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, liquidation preference (in dollars per share)	$ 25	$ 25
Preferred stock, authorized (in shares)	10.0	10.0
Preferred stock, issued (in shares)	10.0	10.0
Preferred stock, outstanding (in shares)	10.0	10.0