Condensed Consolidating Financial Information (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2011	Dec. 31, 2010
Assets
Fixed maturities, available-for-sale	$ 49,501.3	$ 49,006.7
Fixed maturities, trading	868.7	971.7
Equity securities, available-for-sale	138.4	77.1
Equity securities, trading	536.4	404.8
Mortgage loans	11,308.9	10,727.2
Real estate	1,111.9	1,092.9
Policy loans	873.9	885.1
Investment in unconsolidated entities	862.9	827.3
Other investments	2,089.6	2,158.5
Cash and cash equivalents	1,651.9	2,833.9	1,984.2	1,877.4
Accrued investment income	622.5	615.2
Premiums due and other receivables	1,118.8	1,196.5
Deferred policy acquisition costs	2,665.6	2,428.0
Property and equipment	476.7	457.2
Goodwill	490.7	482.3
Other intangibles	893.9	890.6
Separate account assets	77,566.5	71,364.4
Other assets	959.6	942.3
Total assets	153,738.2	147,361.7
Liabilities
Contractholder funds	36,922.5	37,676.4
Future policy benefits and claims	20,604.0	20,210.4
Other policyholder funds	639.1	548.6
Short-term debt	101.2	105.2
Long-term debt	1,570.8	1,564.8
Income taxes currently payable	2.9	3.1
Deferred income taxes	492.8	208.7
Separate account liabilities	77,566.5	71,364.4
Other liabilities	6,086.5	6,286.2
Total liabilities	143,986.3	137,967.8
Stockholders' equity
Common stock	4.5	4.5
Additional paid-in capital	9,669.6	9,634.7
Retained earnings (deficit)	4,548.5	4,402.3
Accumulated other comprehensive income (loss)	488.1	258.0
Treasury stock, at cost (151.5 million and 149.2 million shares in 2012 and 2011)	(5,345.9)	(5,281.7)
Total stockholders' equity attributable to Principal Financial Group, Inc.	9,364.9	9,017.9
Noncontrolling interest	387.0	376.0
Total stockholders' equity	9,751.9	9,393.9	9,786.1	9,306.4
Total liabilities and stockholders' equity	153,738.2	147,361.7
Principal Financial Group, Inc. Parent Only
Assets
Fixed maturities, trading	202.5	268.7
Investment in unconsolidated entities	10,119.1	9,828.0
Other investments	5.8	7.0
Cash and cash equivalents	378.3	226.7	871.8	370.9
Accrued investment income	0.3	1.8
Other assets	14.0	14.8
Total assets	10,720.0	10,347.0
Liabilities
Long-term debt	1,351.7	1,351.7
Income taxes currently payable	(21.7)	(18.6)
Deferred income taxes	(17.6)	(22.5)
Other liabilities	42.7	18.5
Total liabilities	1,355.1	1,329.1
Stockholders' equity
Common stock	4.5	4.5
Additional paid-in capital	9,669.6	9,634.7
Retained earnings (deficit)	4,548.5	4,402.3
Accumulated other comprehensive income (loss)	488.1	258.0
Treasury stock, at cost (151.5 million and 149.2 million shares in 2012 and 2011)	(5,345.9)	(5,281.7)
Total stockholders' equity attributable to Principal Financial Group, Inc.	9,364.9	9,017.9
Total stockholders' equity	9,364.9	9,017.9
Total liabilities and stockholders' equity	10,720.0	10,347.0
Principal Life Insurance Company Only
Assets
Fixed maturities, available-for-sale	43,471.8	43,285.3
Fixed maturities, trading	335.7	374.8
Equity securities, available-for-sale	135.3	73.4
Equity securities, trading	0.3	0.3
Mortgage loans	9,762.1	9,271.5
Real estate	9.1	9.2
Policy loans	845.8	859.3
Investment in unconsolidated entities	3,102.6	3,115.7
Other investments	2,252.5	2,559.0
Cash and cash equivalents	346.6	1,344.5	354.6	699.8
Accrued investment income	557.5	551.1
Premiums due and other receivables	886.8	969.1
Deferred policy acquisition costs	2,409.9	2,197.4
Property and equipment	410.5	395.9
Goodwill	54.3	54.3
Other intangibles	28.9	29.2
Separate account assets	66,708.9	61,615.1
Other assets	464.8	668.9
Total assets	131,783.4	127,374.0
Liabilities
Contractholder funds	36,528.2	37,356.8
Future policy benefits and claims	16,437.2	16,373.3
Other policyholder funds	607.6	519.7
Long-term debt	99.4	99.4
Income taxes currently payable	(400.1)	(218.4)
Deferred income taxes	190.8	90.6
Separate account liabilities	66,708.9	61,615.1
Other liabilities	4,240.0	4,293.3
Total liabilities	124,412.0	120,129.8
Stockholders' equity
Common stock	2.5	2.5
Additional paid-in capital	5,721.7	5,718.1
Retained earnings (deficit)	1,178.1	1,195.0
Accumulated other comprehensive income (loss)	469.1	328.6
Total stockholders' equity attributable to Principal Financial Group, Inc.	7,371.4	7,244.2
Total stockholders' equity	7,371.4	7,244.2
Total liabilities and stockholders' equity	131,783.4	127,374.0
Principal Financial Services, Inc. and Other Subsidiaries Combined
Assets
Fixed maturities, available-for-sale	6,389.7	6,082.4
Fixed maturities, trading	330.5	328.2
Equity securities, available-for-sale	3.1	3.7
Equity securities, trading	536.1	404.5
Mortgage loans	1,921.9	1,831.8
Real estate	1,103.8	1,084.9
Policy loans	28.1	25.8
Investment in unconsolidated entities	4,735.7	4,718.4
Other investments	998.4	925.3
Cash and cash equivalents	941.7	1,277.6	819.6	719.9
Accrued investment income	68.7	66.6
Premiums due and other receivables	935.3	827.7
Deferred policy acquisition costs	255.7	230.6
Property and equipment	66.2	61.3
Goodwill	436.4	428.0
Other intangibles	865.0	861.4
Separate account assets	10,857.6	9,749.3
Other assets	1,001.7	994.7
Total assets	31,475.6	29,902.2
Liabilities
Contractholder funds	664.7	586.7
Future policy benefits and claims	4,281.1	3,937.9
Other policyholder funds	31.7	29.0
Short-term debt	101.2	105.2
Long-term debt	502.4	504.8
Income taxes currently payable	(14.4)	34.3
Deferred income taxes	337.1	155.2
Separate account liabilities	10,857.6	9,749.3
Other liabilities	4,203.6	4,591.5
Total liabilities	20,965.0	19,693.9
Stockholders' equity
Additional paid-in capital	7,881.0	7,870.2
Retained earnings (deficit)	1,711.6	1,660.3
Accumulated other comprehensive income (loss)	526.5	297.5
Total stockholders' equity attributable to Principal Financial Group, Inc.	10,119.1	9,828.0
Noncontrolling interest	391.5	380.3
Total stockholders' equity	10,510.6	10,208.3
Total liabilities and stockholders' equity	31,475.6	29,902.2
Eliminations, Notes Guarantor
Assets
Fixed maturities, available-for-sale	(360.2)	(361.0)
Mortgage loans	(375.1)	(376.1)
Real estate	(1.0)	(1.2)
Investment in unconsolidated entities	(17,094.5)	(16,834.8)
Other investments	(1,167.1)	(1,332.8)
Cash and cash equivalents	(14.7)	(14.9)	(61.8)	86.8
Accrued investment income	(4.0)	(4.3)
Premiums due and other receivables	(703.3)	(600.3)
Other assets	(520.9)	(736.1)
Total assets	(20,240.8)	(20,261.5)
Liabilities
Contractholder funds	(270.4)	(267.1)
Future policy benefits and claims	(114.3)	(100.8)
Other policyholder funds	(0.2)	(0.1)
Long-term debt	(382.7)	(391.1)
Income taxes currently payable	439.1	205.8
Deferred income taxes	(17.5)	(14.6)
Other liabilities	(2,399.8)	(2,617.1)
Total liabilities	(2,745.8)	(3,185.0)
Stockholders' equity
Common stock	(2.5)	(2.5)
Additional paid-in capital	(13,602.7)	(13,588.3)
Retained earnings (deficit)	(2,889.7)	(2,855.3)
Accumulated other comprehensive income (loss)	(995.6)	(626.1)
Total stockholders' equity attributable to Principal Financial Group, Inc.	(17,490.5)	(17,072.2)
Noncontrolling interest	(4.5)	(4.3)
Total stockholders' equity	(17,495.0)	(17,076.5)
Total liabilities and stockholders' equity	(20,240.8)	(20,261.5)
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
Series A
Stockholders' equity
Preferred stock, value