Condensed Consolidating Financial Information (Details) (USD $) In Millions, 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
Equity securities, available-for-sale	77.1	169.9
Equity securities, trading	404.8	316.9
Mortgage loans	10,727.2	11,125.1
Real estate	1,092.9	1,063.5
Policy loans	885.1	903.9
Investment in unconsolidated entities	829.5	736.0
Other investments	2,158.5	1,905.6
Cash and cash equivalents	2,833.9	1,877.4	2,240.4	2,608.0
Accrued investment income	615.2	666.1
Premiums due and other receivables	1,245.2	1,063.0
Deferred policy acquisition costs	3,313.5	3,529.8	3,681.4	4,153.0
Property and equipment	457.2	458.7
Goodwill	482.3	345.4	386.4
Other intangibles	890.6	834.6
Separate account assets	71,364.4	69,555.3
Other assets	942.2	1,323.3
Total assets	148,298.0	145,631.1
Liabilities
Contractholder funds	37,676.4	37,301.1
Future policy benefits and claims	20,207.9	20,046.3
Other policyholder funds	543.7	592.2
Short-term debt	105.2	107.9
Long-term debt	1,564.8	1,583.7
Income taxes currently payable	3.1	6.2
Deferred income taxes	533.4	409.9
Separate account liabilities	71,364.4	69,555.3
Other liabilities	6,286.1	6,143.5
Total liabilities	138,285.0	135,746.1
Stockholders' equity
Common stock	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
Noncontrolling interest	376.0	157.2
Total stockholders' equity	10,013.0	9,885.0	8,016.4	2,569.3
Total liabilities and stockholders' equity	148,298.0	145,631.1
Principal Financial Group, Inc. Parent Only
Assets
Fixed maturities, available-for-sale		199.9
Fixed maturities, trading	268.7	274.9
Investment in unconsolidated entities	10,447.1	10,195.1
Other investments	7.0	5.6
Cash and cash equivalents	226.7	370.9	304.6	(2.0)
Accrued investment income	1.8	0.8
Other assets	14.8	13.6
Total assets	10,966.1	11,060.8
Liabilities
Long-term debt	1,351.7	1,351.7
Income taxes currently payable	(18.6)	(18.8)
Deferred income taxes	(22.5)	(21.6)
Other liabilities	18.5	21.7
Total liabilities	1,329.1	1,333.0
Stockholders' equity
Common stock	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 stockholders' equity	9,637.0	9,727.8
Total liabilities and stockholders' equity	10,966.1	11,060.8
Principal Life Insurance Company Only
Assets
Fixed maturities, available-for-sale	43,285.3	42,478.2
Fixed maturities, trading	374.8	475.8
Equity securities, available-for-sale	73.4	165.8
Equity securities, trading	0.3	0.3
Mortgage loans	9,271.5	9,466.9
Real estate	9.2	9.6
Policy loans	859.3	878.3
Investment in unconsolidated entities	3,116.1	3,433.6
Other investments	2,559.0	1,603.8
Cash and cash equivalents	1,344.5	699.8	1,249.2	1,598.6
Accrued investment income	551.1	607.4
Premiums due and other receivables	1,017.8	862.6
Deferred policy acquisition costs	3,034.5	3,258.7
Property and equipment	395.9	389.6
Goodwill	54.3	54.3
Other intangibles	29.2	30.5
Separate account assets	61,615.1	62,738.4
Other assets	668.9	716.0
Total assets	128,260.2	127,869.6
Liabilities
Contractholder funds	37,356.8	37,353.3
Future policy benefits and claims	16,370.8	16,082.5
Other policyholder funds	514.8	569.2
Long-term debt	99.4	99.5
Income taxes currently payable	(218.4)	(188.0)
Deferred income taxes	403.3	134.8
Separate account liabilities	61,615.1	62,738.4
Other liabilities	4,293.8	3,264.6
Total liabilities	120,435.6	120,054.3
Stockholders' equity
Common stock	2.5	2.5
Additional paid-in capital	5,718.1	6,145.0
Retained earnings (deficit)	1,826.0	1,472.4
Accumulated other comprehensive income (loss)	278.0	195.4
Total stockholders' equity attributable to Principal Financial Group, Inc.	7,824.6	7,815.3
Total stockholders' equity	7,824.6	7,815.3
Total liabilities and stockholders' equity	128,260.2	127,869.6
Principal Financial Services, Inc. and Other Subsidiaries Combined
Assets
Fixed maturities, available-for-sale	6,082.4	6,381.9
Fixed maturities, trading	328.2	369.6
Equity securities, available-for-sale	3.7	4.1
Equity securities, trading	404.5	316.6
Mortgage loans	1,831.8	2,072.3
Real estate	1,084.9	1,055.6
Policy loans	25.8	25.6
Investment in unconsolidated entities	5,300.5	4,841.9
Other investments	925.3	769.0
Cash and cash equivalents	1,277.6	719.9	713.0	1,192.3
Accrued investment income	66.6	62.5
Premiums due and other receivables	827.7	405.4
Deferred policy acquisition costs	279.0	271.1
Property and equipment	61.3	69.1
Goodwill	428.0	291.1
Other intangibles	861.4	804.1
Separate account assets	9,749.3	6,816.9
Other assets	994.6	1,146.3
Total assets	30,532.6	26,423.0
Liabilities
Contractholder funds	586.7	208.9
Future policy benefits and claims	3,937.9	4,013.3
Other policyholder funds	29.0	23.0
Short-term debt	105.2	107.9
Long-term debt	504.8	581.0
Income taxes currently payable	34.3	28.5
Deferred income taxes	167.2	309.8
Separate account liabilities	9,749.3	6,816.9
Other liabilities	4,590.8	3,977.1
Total liabilities	19,705.2	16,066.4
Stockholders' equity
Additional paid-in capital	7,870.2	8,334.0
Retained earnings (deficit)	2,335.6	1,546.3
Accumulated other comprehensive income (loss)	241.3	314.8
Total stockholders' equity attributable to Principal Financial Group, Inc.	10,447.1	10,195.1
Noncontrolling interest	380.3	161.5
Total stockholders' equity	10,827.4	10,356.6
Total liabilities and stockholders' equity	30,532.6	26,423.0
Eliminations, Notes Guarantor
Assets
Fixed maturities, available-for-sale	(361.0)	(423.7)
Mortgage loans	(376.1)	(414.1)
Real estate	(1.2)	(1.7)
Investment in unconsolidated entities	(18,034.2)	(17,734.6)
Other investments	(1,332.8)	(472.8)
Cash and cash equivalents	(14.9)	86.8	(26.4)	(180.9)
Accrued investment income	(4.3)	(4.6)
Premiums due and other receivables	(600.3)	(205.0)
Other assets	(736.1)	(552.6)
Total assets	(21,460.9)	(19,722.3)
Liabilities
Contractholder funds	(267.1)	(261.1)
Future policy benefits and claims	(100.8)	(49.5)
Other policyholder funds	(0.1)
Long-term debt	(391.1)	(448.5)
Income taxes currently payable	205.8	184.5
Deferred income taxes	(14.6)	(13.1)
Other liabilities	(2,617.0)	(1,119.9)
Total liabilities	(3,184.9)	(1,707.6)
Stockholders' equity
Common stock	(2.5)	(2.5)
Additional paid-in capital	(13,588.3)	(14,479.0)
Retained earnings (deficit)	(4,161.6)	(3,018.7)
Accumulated other comprehensive income (loss)	(519.3)	(510.2)
Total stockholders' equity attributable to Principal Financial Group, Inc.	(18,271.7)	(18,010.4)
Noncontrolling interest	(4.3)	(4.3)
Total stockholders' equity	(18,276.0)	(18,014.7)
Total liabilities and stockholders' equity	(21,460.9)	(19,722.3)
Series A
Stockholders' equity
Preferred stock, value
Series B
Stockholders' equity
Preferred stock, value	0.1	0.1
Series B | Principal Financial Group, Inc. Parent Only
Stockholders' equity
Preferred stock, value	$ 0.1	$ 0.1