Condensed Consolidating Financial Information (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011
Assets
Fixed maturities, available-for-sale	$ 51,084.1	$ 50,939.3
Fixed maturities, trading	628.9	626.7
Equity securities, available-for-sale	147.0	136.5
Equity securities, trading	732.2	252.8
Mortgage loans	11,577.9	11,519.7
Real estate	1,202.0	1,180.3
Policy loans	866.5	864.9
Investment in unconsolidated entities	881.4	869.0
Other investments	2,305.8	2,422.1
Cash and cash equivalents	1,661.2	4,177.2	1,651.9	2,833.9
Accrued investment income	584.5	584.4
Premiums due and other receivables	1,175.2	1,084.4
Deferred acquisition costs	2,715.5	2,590.0
Property and equipment	472.6	464.2
Goodwill	1,197.4	543.4		482.3
Other intangibles	1,590.3	914.7
Separate account assets	120,906.1	81,653.8
Other assets	1,034.3	1,006.8
Total assets	200,762.9	161,830.2
Liabilities
Contractholder funds	36,701.6	37,786.5
Future policy benefits and claims	22,689.5	22,436.2
Other policyholder funds	811.2	716.4
Short-term debt	46.5	40.8
Long-term debt	2,790.5	2,671.3
Income taxes currently payable	9.8	15.3
Deferred income taxes	791.4	600.0
Separate account liabilities	120,906.1	81,653.8
Other liabilities	6,130.8	6,146.1
Total liabilities	190,877.4	152,066.4
Redeemable noncontrolling interest	58.9	60.4	23.1	22.2
Stockholders' equity
Common stock	4.6	4.5
Additional paid-in capital	9,771.5	9,730.9
Retained earnings (accumulated deficit)	4,971.7	4,862.0
Accumulated other comprehensive income (loss)	621.0	640.3
Treasury stock, at cost (162.6 million and 159.7 million shares in 2013 and 2012)	(5,645.3)	(5,554.4)
Total stockholders' equity attributable to Principal Financial Group, Inc.	9,723.6	9,683.4
Noncontrolling interest	103.0	20.0
Total stockholders' equity	9,826.6	9,703.4	9,660.0	9,306.2
Total liabilities and stockholders' equity	200,762.9	161,830.2
Principal Financial Group, Inc. Parent Only
Assets
Fixed maturities, trading		10.5
Investment in unconsolidated entities	12,036.1	11,853.3
Other investments	9.0	11.1
Cash and cash equivalents	112.7	207.1	378.3	226.7
Premiums due and other receivables		0.1
Other assets	66.2	78.0
Total assets	12,224.0	12,160.1
Liabilities
Long-term debt	2,448.6	2,448.6
Other liabilities	51.8	28.1
Total liabilities	2,500.4	2,476.7
Stockholders' equity
Common stock	4.6	4.5
Additional paid-in capital	9,771.5	9,730.9
Retained earnings (accumulated deficit)	4,971.7	4,862.0
Accumulated other comprehensive income (loss)	621.0	640.3
Treasury stock, at cost (162.6 million and 159.7 million shares in 2013 and 2012)	(5,645.3)	(5,554.4)
Total stockholders' equity attributable to Principal Financial Group, Inc.	9,723.6	9,683.4
Total stockholders' equity	9,723.6	9,683.4
Total liabilities and stockholders' equity	12,224.0	12,160.1
Principal Life Insurance Company Only
Assets
Fixed maturities, available-for-sale	44,789.5	44,614.6
Fixed maturities, trading	279.2	284.4
Equity securities, available-for-sale	130.0	131.3
Equity securities, trading	0.3	0.3
Mortgage loans	10,107.7	10,054.2
Real estate	8.3	8.4
Policy loans	833.5	834.0
Investment in unconsolidated entities	3,321.2	3,309.2
Other investments	2,504.8	2,834.0
Cash and cash equivalents	711.1	1,698.4	346.6	1,344.5
Accrued investment income	518.2	521.6
Premiums due and other receivables	1,020.6	916.7
Deferred acquisition costs	2,504.7	2,394.8
Property and equipment	395.2	402.2
Goodwill	54.3	54.3
Other intangibles	27.6	27.9
Separate account assets	73,423.6	69,217.8
Other assets	983.3	947.8
Total assets	141,613.1	138,251.9
Liabilities
Contractholder funds	35,868.5	37,053.3
Future policy benefits and claims	18,048.2	17,944.9
Other policyholder funds	767.4	676.5
Long-term debt	99.4	99.4
Deferred income taxes	230.6	324.5
Separate account liabilities	73,423.6	69,217.8
Other liabilities	5,531.5	5,375.1
Total liabilities	133,969.2	130,691.5
Stockholders' equity
Common stock	2.5	2.5
Additional paid-in capital	5,745.0	5,747.6
Retained earnings (accumulated deficit)	1,318.0	1,167.7
Accumulated other comprehensive income (loss)	578.4	642.6
Total stockholders' equity attributable to Principal Financial Group, Inc.	7,643.9	7,560.4
Total stockholders' equity	7,643.9	7,560.4
Total liabilities and stockholders' equity	141,613.1	138,251.9
Principal Financial Services, Inc. and Other Subsidiaries Combined
Assets
Fixed maturities, available-for-sale	6,661.0	6,681.7
Fixed maturities, trading	349.7	331.8
Equity securities, available-for-sale	17.0	5.2
Equity securities, trading	731.9	252.5
Mortgage loans	1,801.3	1,775.5
Real estate	1,193.7	1,171.9
Policy loans	33.0	30.9
Investment in unconsolidated entities	4,666.1	4,808.3
Other investments	1,358.1	1,208.4
Cash and cash equivalents	751.9	2,286.9	941.7	1,277.6
Accrued investment income	68.0	64.5
Premiums due and other receivables	1,460.9	1,327.1
Deferred acquisition costs	210.8	195.2
Property and equipment	77.4	62.0
Goodwill	1,143.1	489.1
Other intangibles	1,562.7	886.8
Separate account assets	47,482.5	12,436.0
Other assets	1,660.8	1,567.7
Total assets	71,229.9	35,581.5
Liabilities
Contractholder funds	1,113.9	1,011.9
Future policy benefits and claims	4,854.2	4,679.6
Other policyholder funds	44.1	40.3
Short-term debt	46.5	40.8
Long-term debt	573.6	433.3
Income taxes currently payable	82.2	84.7
Deferred income taxes	686.8	378.4
Separate account liabilities	47,482.5	12,436.0
Other liabilities	4,143.6	4,538.4
Total liabilities	59,027.4	23,643.4
Redeemable noncontrolling interest	58.9	60.4
Stockholders' equity
Additional paid-in capital	9,394.8	9,393.8
Retained earnings (accumulated deficit)	1,986.1	1,783.4
Accumulated other comprehensive income (loss)	655.2	676.1
Total stockholders' equity attributable to Principal Financial Group, Inc.	12,036.1	11,853.3
Noncontrolling interest	107.5	24.4
Total stockholders' equity	12,143.6	11,877.7
Total liabilities and stockholders' equity	71,229.9	35,581.5
Eliminations, Notes Guarantor
Assets
Fixed maturities, available-for-sale	(366.4)	(357.0)
Mortgage loans	(331.1)	(310.0)
Investment in unconsolidated entities	(19,142.0)	(19,101.8)
Other investments	(1,566.1)	(1,631.4)
Cash and cash equivalents	85.5	(15.2)	(14.7)	(14.9)
Accrued investment income	(1.7)	(1.7)
Premiums due and other receivables	(1,306.3)	(1,159.5)
Other assets	(1,676.0)	(1,586.7)
Total assets	(24,304.1)	(24,163.3)
Liabilities
Contractholder funds	(280.8)	(278.7)
Future policy benefits and claims	(212.9)	(188.3)
Other policyholder funds	(0.3)	(0.4)
Long-term debt	(331.1)	(310.0)
Income taxes currently payable	(72.4)	(69.4)
Deferred income taxes	(126.0)	(102.9)
Other liabilities	(3,596.1)	(3,795.5)
Total liabilities	(4,619.6)	(4,745.2)
Stockholders' equity
Common stock	(2.5)	(2.5)
Additional paid-in capital	(15,139.8)	(15,141.4)
Retained earnings (accumulated deficit)	(3,304.1)	(2,951.1)
Accumulated other comprehensive income (loss)	(1,233.6)	(1,318.7)
Total stockholders' equity attributable to Principal Financial Group, Inc.	(19,680.0)	(19,413.7)
Noncontrolling interest	(4.5)	(4.4)
Total stockholders' equity	(19,684.5)	(19,418.1)
Total liabilities and stockholders' equity	(24,304.1)	(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