Condensed Consolidating Financial Information (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012	Dec. 31, 2011
Assets
Fixed maturities, available-for-sale	$ 49,191.6	$ 50,939.3
Fixed maturities, trading	589.8	626.7
Equity securities, available-for-sale	135.3	136.5
Equity securities, trading	636.7	252.8
Mortgage loans	12,121.7	11,519.7
Real estate	1,264.8	1,180.3
Policy loans	865.3	864.9
Investment in unconsolidated entities	879.3	869.0
Other investments	2,097.0	2,422.1
Cash and cash equivalents	1,110.5	4,177.2	1,646.6	2,833.9
Accrued investment income	565.4	584.4
Premiums due and other receivables	1,219.0	1,084.4
Deferred acquisition costs	2,925.4	2,590.0
Property and equipment	476.4	464.2
Goodwill	1,138.5	543.4
Other intangibles	1,517.3	914.7
Separate account assets	118,740.2	81,653.8
Other assets	1,038.5	1,006.8
Total assets	196,512.7	161,830.2
Liabilities
Contractholder funds	36,269.7	37,786.5
Future policy benefits and claims	22,129.5	22,436.2
Other policyholder funds	739.5	716.4
Short-term debt	175.3	40.8
Long-term debt	2,578.6	2,671.3
Income taxes currently payable	7.2	15.3
Deferred income taxes	578.7	600.0
Separate account liabilities	118,740.2	81,653.8
Other liabilities	5,831.5	6,146.1
Total liabilities	187,050.2	152,066.4
Redeemable noncontrolling interest	53.6	60.4	60.5	22.2
Stockholders' equity
Common stock	4.6	4.5
Additional paid-in capital	9,819.8	9,730.9
Retained earnings (accumulated deficit)	5,125.2	4,862.0
Accumulated other comprehensive income (loss)	(0.5)	640.3
Treasury stock, at cost (162.6 million and 159.7 million shares in 2013 and 2012)	(5,645.8)	(5,554.4)
Total stockholders' equity attributable to Principal Financial Group, Inc.	9,303.4	9,683.4
Noncontrolling interest	105.5	20.0
Total stockholders' equity	9,408.9	9,703.4	9,363.8	9,306.2
Total liabilities and stockholders' equity	196,512.7	161,830.2
Principal Financial Group, Inc. Parent Only
Assets
Fixed maturities, trading		10.5
Investment in unconsolidated entities	11,693.7	11,853.3
Other investments	9.0	11.1
Cash and cash equivalents	18.5	207.1	374.4	226.7
Premiums due and other receivables		0.1
Other assets	64.1	78.0
Total assets	11,785.3	12,160.1
Liabilities
Long-term debt	2,448.7	2,448.6
Other liabilities	33.2	28.1
Total liabilities	2,481.9	2,476.7
Stockholders' equity
Common stock	4.6	4.5
Additional paid-in capital	9,819.8	9,730.9
Retained earnings (accumulated deficit)	5,125.2	4,862.0
Accumulated other comprehensive income (loss)	(0.5)	640.3
Treasury stock, at cost (162.6 million and 159.7 million shares in 2013 and 2012)	(5,645.8)	(5,554.4)
Total stockholders' equity attributable to Principal Financial Group, Inc.	9,303.4	9,683.4
Total stockholders' equity	9,303.4	9,683.4
Total liabilities and stockholders' equity	11,785.3	12,160.1
Principal Life Insurance Company Only
Assets
Fixed maturities, available-for-sale	43,235.9	44,614.6
Fixed maturities, trading	263.3	284.4
Equity securities, available-for-sale	119.0	131.3
Equity securities, trading	0.3	0.3
Mortgage loans	10,787.0	10,054.2
Real estate	8.1	8.4
Policy loans	834.5	834.0
Investment in unconsolidated entities	3,278.0	3,309.2
Other investments	2,322.5	2,834.0
Cash and cash equivalents	211.7	1,698.4	472.8	1,344.5
Accrued investment income	502.1	521.6
Premiums due and other receivables	1,044.5	916.7
Deferred acquisition costs	2,714.7	2,394.8
Property and equipment	394.4	402.2
Goodwill	54.3	54.3
Other intangibles	27.4	27.9
Separate account assets	73,199.8	69,217.8
Other assets	971.8	947.8
Total assets	139,969.3	138,251.9
Liabilities
Contractholder funds	35,370.8	37,053.3
Future policy benefits and claims	17,799.0	17,944.9
Other policyholder funds	692.8	676.5
Long-term debt	99.4	99.4
Deferred income taxes	73.4	324.5
Separate account liabilities	73,199.8	69,217.8
Other liabilities	5,286.0	5,375.1
Total liabilities	132,521.2	130,691.5
Stockholders' equity
Common stock	2.5	2.5
Additional paid-in capital	5,769.4	5,747.6
Retained earnings (accumulated deficit)	1,496.3	1,167.7
Accumulated other comprehensive income (loss)	179.9	642.6
Total stockholders' equity attributable to Principal Financial Group, Inc.	7,448.1	7,560.4
Total stockholders' equity	7,448.1	7,560.4
Total liabilities and stockholders' equity	139,969.3	138,251.9
Principal Financial Services, Inc. and Other Subsidiaries Combined
Assets
Fixed maturities, available-for-sale	6,341.5	6,681.7
Fixed maturities, trading	326.5	331.8
Equity securities, available-for-sale	16.3	5.2
Equity securities, trading	636.4	252.5
Mortgage loans	1,667.1	1,775.5
Real estate	1,256.7	1,171.9
Policy loans	30.8	30.9
Investment in unconsolidated entities	4,573.0	4,808.3
Other investments	1,264.3	1,208.4
Cash and cash equivalents	700.4	2,286.9	826.1	1,277.6
Accrued investment income	64.9	64.5
Premiums due and other receivables	1,577.4	1,327.1
Deferred acquisition costs	210.7	195.2
Property and equipment	82.0	62.0
Goodwill	1,084.2	489.1
Other intangibles	1,489.9	886.8
Separate account assets	45,540.4	12,436.0
Other assets	1,705.4	1,567.7
Total assets	68,567.9	35,581.5
Liabilities
Contractholder funds	1,183.2	1,011.9
Future policy benefits and claims	4,565.6	4,679.6
Other policyholder funds	47.1	40.3
Short-term debt	175.3	40.8
Long-term debt	362.9	433.3
Income taxes currently payable	91.7	84.7
Deferred income taxes	666.0	378.4
Separate account liabilities	45,540.4	12,436.0
Other liabilities	4,078.4	4,538.4
Total liabilities	56,710.6	23,643.4
Redeemable noncontrolling interest	53.6	60.4
Stockholders' equity
Additional paid-in capital	9,424.5	9,393.8
Retained earnings (accumulated deficit)	2,236.6	1,783.4
Accumulated other comprehensive income (loss)	32.6	676.1
Total stockholders' equity attributable to Principal Financial Group, Inc.	11,693.7	11,853.3
Noncontrolling interest	110.0	24.4
Total stockholders' equity	11,803.7	11,877.7
Total liabilities and stockholders' equity	68,567.9	35,581.5
Eliminations, Notes Guarantor
Assets
Fixed maturities, available-for-sale	(385.8)	(357.0)
Mortgage loans	(332.4)	(310.0)
Investment in unconsolidated entities	(18,665.4)	(19,101.8)
Other investments	(1,498.8)	(1,631.4)
Cash and cash equivalents	179.9	(15.2)	(26.7)	(14.9)
Accrued investment income	(1.6)	(1.7)
Premiums due and other receivables	(1,402.9)	(1,159.5)
Other assets	(1,702.8)	(1,586.7)
Total assets	(23,809.8)	(24,163.3)
Liabilities
Contractholder funds	(284.3)	(278.7)
Future policy benefits and claims	(235.1)	(188.3)
Other policyholder funds	(0.4)	(0.4)
Long-term debt	(332.4)	(310.0)
Income taxes currently payable	(84.5)	(69.4)
Deferred income taxes	(160.7)	(102.9)
Other liabilities	(3,566.1)	(3,795.5)
Total liabilities	(4,663.5)	(4,745.2)
Stockholders' equity
Common stock	(2.5)	(2.5)
Additional paid-in capital	(15,193.9)	(15,141.4)
Retained earnings (accumulated deficit)	(3,732.9)	(2,951.1)
Accumulated other comprehensive income (loss)	(212.5)	(1,318.7)
Total stockholders' equity attributable to Principal Financial Group, Inc.	(19,141.8)	(19,413.7)
Noncontrolling interest	(4.5)	(4.4)
Total stockholders' equity	(19,146.3)	(19,418.1)
Total liabilities and stockholders' equity	(23,809.8)	(24,163.3)
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