Condensed Consolidating Financial Information (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Assets
Fixed maturities, available-for-sale	$ 49,670.8	$ 48,757.1
Fixed maturities, trading	604.6	563.1
Equity securities, available-for-sale	123.0	110.5
Equity securities, trading	840.2	716.9
Mortgage loans	11,811.6	11,533.6
Real estate	1,344.6	1,271.6
Policy loans	829.2	859.7
Investment in unconsolidated entities	815.8	880.0
Other investments	2,394.0	2,064.4
Cash and cash equivalents	1,863.9	2,371.8	4,177.2	2,833.9
Accrued investment income	505.9	532.1
Premiums due and other receivables	1,213.0	1,241.0
Deferred acquisition costs	2,993.0	3,077.0	2,590.0	2,358.1
Property and equipment	590.2	500.7
Goodwill	1,007.4	1,100.3	543.4
Other intangibles	1,323.5	1,459.0
Separate account assets	140,072.8	130,018.4
Other assets	1,083.5	1,134.2
Total assets	219,087.0	208,191.4
Liabilities
Contractholder funds	34,726.7	35,958.3
Future policy benefits and claims	24,036.6	22,626.2
Other policyholder funds	812.7	758.9
Short-term debt	28.0	150.6
Long-term debt	2,531.2	2,601.4
Income taxes currently payable	11.5	5.2
Deferred income taxes	1,035.3	824.0
Separate account liabilities	140,072.8	130,018.4
Other liabilities	5,542.2	5,224.2
Total liabilities	208,797.0	198,167.2
Redeemable noncontrolling interest	58.0	247.2	60.4	22.2
Stockholders' equity
Common stock	4.6	4.6
Additional paid-in capital	9,945.5	9,798.9
Retained earnings (accumulated deficit)	6,114.1	5,405.4
Accumulated other comprehensive income	50.4	183.2
Treasury stock, at cost	(5,930.7)	(5,708.0)
Total stockholders' equity attributable to Principal Financial Group, Inc.	10,184.0	9,684.2
Noncontrolling interest	48.0	92.8
Total stockholders' equity	10,232.0	9,777.0	9,703.4	9,306.2
Total liabilities and stockholders' equity	219,087.0	208,191.4
Principal Financial Group, Inc. Parent Only
Assets
Investment in unconsolidated entities	12,446.5	11,956.2
Other investments	9.5	9.3
Cash and cash equivalents	412.4	131.5	207.1	226.7
Other assets	450.7	59.2
Total assets	13,319.1	12,156.2
Liabilities
Long-term debt	2,448.9	2,448.8
Other liabilities	686.2	23.2
Total liabilities	3,135.1	2,472.0
Stockholders' equity
Common stock	4.6	4.6
Additional paid-in capital	9,945.5	9,798.9
Retained earnings (accumulated deficit)	6,114.1	5,405.4
Accumulated other comprehensive income	50.4	183.2
Treasury stock, at cost	(5,930.7)	(5,708.0)
Total stockholders' equity attributable to Principal Financial Group, Inc.	10,184.0	9,684.2
Total stockholders' equity	10,184.0	9,684.2
Total liabilities and stockholders' equity	13,319.1	12,156.2
Principal Life Insurance Company Only
Assets
Fixed maturities, available-for-sale	43,649.7	42,794.7
Fixed maturities, trading	298.5	245.5
Equity securities, available-for-sale	108.9	102.6
Equity securities, trading	0.3	0.3
Mortgage loans	10,972.6	10,501.5
Real estate	7.0	7.9
Policy loans	799.0	830.1
Investment in unconsolidated entities	2,787.9	3,396.8
Other investments	3,416.7	1,892.4
Cash and cash equivalents	602.7	1,332.2	1,698.4	1,344.5
Accrued investment income	449.2	474.5
Premiums due and other receivables	1,237.4	1,029.0
Deferred acquisition costs	2,754.6	2,848.8
Property and equipment	505.8	422.1
Goodwill	54.3	54.3
Other intangibles	25.7	26.9
Separate account assets	94,328.4	83,790.2
Other assets	873.2	976.9
Total assets	162,871.9	150,726.7
Liabilities
Contractholder funds	33,428.3	34,918.0
Future policy benefits and claims	19,768.1	18,292.9
Other policyholder funds	742.8	705.1
Long-term debt		99.4
Deferred income taxes	629.9	(25.0)
Separate account liabilities	94,328.4	83,790.2
Other liabilities	5,793.0	5,204.8
Total liabilities	154,690.5	142,985.4
Stockholders' equity
Common stock	2.5	2.5
Additional paid-in capital	5,275.0	5,505.0
Retained earnings (accumulated deficit)	1,817.2	1,738.1
Accumulated other comprehensive income	1,086.7	495.7
Total stockholders' equity attributable to Principal Financial Group, Inc.	8,181.4	7,741.3
Total stockholders' equity	8,181.4	7,741.3
Total liabilities and stockholders' equity	162,871.9	150,726.7
Principal Financial Services, Inc. and Other Subsidiaries Combined
Assets
Fixed maturities, available-for-sale	6,411.6	6,357.5
Fixed maturities, trading	306.1	317.6
Equity securities, available-for-sale	14.1	7.9
Equity securities, trading	839.9	716.6
Mortgage loans	1,188.6	1,345.9
Real estate	1,337.6	1,263.7
Policy loans	30.2	29.6
Investment in unconsolidated entities	5,745.7	4,891.6
Other investments	1,578.5	1,238.8
Cash and cash equivalents	1,253.6	894.5	2,286.9	1,277.6
Accrued investment income	58.1	59.0
Premiums due and other receivables	2,165.6	1,814.5
Deferred acquisition costs	238.4	228.2
Property and equipment	84.4	78.6
Goodwill	953.1	1,046.0
Other intangibles	1,297.8	1,432.1
Separate account assets	45,744.4	46,228.2
Other assets	2,768.6	2,115.3
Total assets	72,016.3	70,065.6
Liabilities
Contractholder funds	1,607.5	1,330.7
Future policy benefits and claims	4,718.2	4,625.0
Other policyholder funds	70.4	54.2
Short-term debt	28.0	150.6
Long-term debt	417.5	367.0
Income taxes currently payable	83.1	67.4
Deferred income taxes	1,022.5	1,030.1
Separate account liabilities	45,744.4	46,228.2
Other liabilities	5,845.5	3,911.8
Total liabilities	59,537.1	57,765.0
Redeemable noncontrolling interest	58.0	247.2
Stockholders' equity
Additional paid-in capital	8,964.2	9,163.7
Retained earnings (accumulated deficit)	2,858.6	2,578.2
Accumulated other comprehensive income	545.9	214.3
Total stockholders' equity attributable to Principal Financial Group, Inc.	12,368.7	11,956.2
Noncontrolling interest	52.5	97.2
Total stockholders' equity	12,421.2	12,053.4
Total liabilities and stockholders' equity	72,016.3	70,065.6
Eliminations, Notes Guarantor
Assets
Fixed maturities, available-for-sale	(390.5)	(395.1)
Mortgage loans	(349.6)	(313.8)
Investment in unconsolidated entities	(20,164.3)	(19,364.6)
Other investments	(2,610.7)	(1,076.1)
Cash and cash equivalents	(404.8)	13.6	(15.2)	(14.9)
Accrued investment income	(1.4)	(1.4)
Premiums due and other receivables	(2,190.0)	(1,602.5)
Other assets	(3,009.0)	(2,017.2)
Total assets	(29,120.3)	(24,757.1)
Liabilities
Contractholder funds	(309.1)	(290.4)
Future policy benefits and claims	(449.7)	(291.7)
Other policyholder funds	(0.5)	(0.4)
Long-term debt	(335.2)	(313.8)
Income taxes currently payable	(71.6)	(62.2)
Deferred income taxes	(617.1)	(181.1)
Other liabilities	(6,782.5)	(3,915.6)
Total liabilities	(8,565.7)	(5,055.2)
Stockholders' equity
Common stock	(2.5)	(2.5)
Additional paid-in capital	(14,239.2)	(14,668.7)
Retained earnings (accumulated deficit)	(4,675.8)	(4,316.3)
Accumulated other comprehensive income	(1,632.6)	(710.0)
Total stockholders' equity attributable to Principal Financial Group, Inc.	(20,550.1)	(19,697.5)
Noncontrolling interest	(4.5)	(4.4)
Total stockholders' equity	(20,554.6)	(19,701.9)
Total liabilities and stockholders' equity	(29,120.3)	(24,757.1)
Series A
Stockholders' equity
Preferred stock, value	0	0
Total stockholders' equity	0	0	0	0
Series B
Stockholders' equity
Preferred stock, value	0.1	0.1
Total stockholders' equity	0.1	0.1	0.1	0.1
Series B | Principal Financial Group, Inc. Parent Only
Stockholders' equity
Preferred stock, value	$ 0.1	$ 0.1