Condensed Consolidating Financial Information (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2013
Assets
Fixed maturities, available-for-sale	$ 49,780.6	$ 49,670.8
Fixed maturities, trading	710.2	604.6
Equity securities, available-for-sale	465.3	123.0
Equity securities, trading	869.1	840.2
Mortgage loans	12,232.1	11,811.6
Real estate	1,415.0	1,344.6
Policy loans	822.8	829.2
Investment in unconsolidated entities	590.4	815.8
Other investments	2,680.0	2,394.0
Cash and cash equivalents	2,514.2	1,863.9	$ 1,270.8	$ 2,371.8
Accrued investment income	566.8	505.9
Premiums due and other receivables	1,401.6	1,213.0
Deferred acquisition costs	3,145.0	2,993.0
Property and equipment	611.6	590.2
Goodwill	1,005.2	1,007.4
Other intangibles	1,398.9	1,323.5
Separate account assets	134,045.8	140,072.8
Other assets	1,110.8	1,083.5
Total assets	215,365.4	219,087.0
Liabilities
Contractholder funds	35,115.2	34,726.7
Future policy benefits and claims	25,521.7	24,036.6
Other policyholder funds	819.5	812.7
Short-term debt	221.6	28.0
Long-term debt	3,286.1	2,531.2
Income taxes currently payable	16.7	11.5
Deferred income taxes	878.8	1,035.3
Separate account liabilities	134,045.8	140,072.8
Other liabilities	5,696.2	5,542.2
Total liabilities	205,601.6	208,797.0
Redeemable noncontrolling interest	78.0	58.0	56.1	247.2
Stockholders' equity
Common stock	4.7	4.6
Additional paid-in capital	9,524.2	9,945.5
Retained earnings (accumulated deficit)	6,734.7	6,114.1
Accumulated other comprehensive income	(524.4)	50.4
Treasury stock, at cost	(6,118.9)	(5,930.7)
Total stockholders' equity attributable to Principal Financial Group, Inc.	9,620.3	10,184.0
Noncontrolling interest	65.5	48.0
Total stockholders' equity	9,685.8	10,232.0	10,369.0	9,777.0
Total liabilities and stockholders' equity	215,365.4	219,087.0
Principal Financial Group, Inc. Parent Only
Assets
Investment in unconsolidated entities	12,494.0	12,446.5
Other investments	9.7	9.5
Cash and cash equivalents	578.1	412.4	150.7	131.5
Other assets	452.7	450.7
Total assets	13,534.5	13,319.1
Liabilities
Long-term debt	3,247.9	2,448.9
Other liabilities	666.3	686.2
Total liabilities	3,914.2	3,135.1
Stockholders' equity
Common stock	4.7	4.6
Additional paid-in capital	9,524.2	9,945.5
Retained earnings (accumulated deficit)	6,734.7	6,114.1
Accumulated other comprehensive income	(524.4)	50.4
Treasury stock, at cost	(6,118.9)	(5,930.7)
Total stockholders' equity attributable to Principal Financial Group, Inc.	9,620.3	10,184.0
Total stockholders' equity	9,620.3	10,184.0
Total liabilities and stockholders' equity	13,534.5	13,319.1
Principal Life Insurance Company Only
Assets
Fixed maturities, available-for-sale	43,723.9	43,649.7
Fixed maturities, trading	358.7	298.5
Equity securities, available-for-sale	101.5	108.9
Equity securities, trading	0.3	0.3
Mortgage loans	11,527.0	10,972.6
Real estate	6.4	7.0
Policy loans	792.7	799.0
Investment in unconsolidated entities	2,701.4	2,787.9
Other investments	3,827.7	3,416.7
Cash and cash equivalents	928.6	602.7	149.1	1,332.2
Accrued investment income	500.3	449.2
Premiums due and other receivables	1,455.3	1,237.4
Deferred acquisition costs	2,925.9	2,754.6
Property and equipment	530.6	505.8
Goodwill	54.3	54.3
Other intangibles	24.9	25.7
Separate account assets	92,088.8	94,328.4
Other assets	913.8	873.2
Total assets	162,462.1	162,871.9
Liabilities
Contractholder funds	32,593.0	33,428.3
Future policy benefits and claims	21,564.0	19,768.1
Other policyholder funds	737.9	742.8
Income taxes currently payable	10.1
Deferred income taxes	465.9	629.9
Separate account liabilities	92,088.8	94,328.4
Other liabilities	6,480.9	5,793.0
Total liabilities	153,940.6	154,690.5
Stockholders' equity
Common stock	2.5	2.5
Additional paid-in capital	5,322.2	5,275.0
Retained earnings (accumulated deficit)	2,289.7	1,817.2
Accumulated other comprehensive income	907.1	1,086.7
Total stockholders' equity attributable to Principal Financial Group, Inc.	8,521.5	8,181.4
Total stockholders' equity	8,521.5	8,181.4
Total liabilities and stockholders' equity	162,462.1	162,871.9
Principal Financial Services, Inc. and Other Subsidiaries Combined
Assets
Fixed maturities, available-for-sale	6,433.3	6,411.6
Fixed maturities, trading	351.5	306.1
Equity securities, available-for-sale	363.8	14.1
Equity securities, trading	868.8	839.9
Mortgage loans	1,174.3	1,188.6
Real estate	1,408.6	1,337.6
Policy loans	30.1	30.2
Investment in unconsolidated entities	5,957.4	5,745.7
Other investments	1,573.9	1,578.5
Cash and cash equivalents	1,420.0	1,253.6	833.5	894.5
Accrued investment income	68.2	58.1
Premiums due and other receivables	2,390.3	2,165.6
Deferred acquisition costs	219.1	238.4
Property and equipment	81.0	84.4
Goodwill	950.9	953.1
Other intangibles	1,374.0	1,297.8
Separate account assets	41,957.0	45,744.4
Other assets	3,093.7	2,768.6
Total assets	69,715.9	72,016.3
Liabilities
Contractholder funds	2,838.1	1,607.5
Future policy benefits and claims	4,461.2	4,718.2
Other policyholder funds	82.6	70.4
Short-term debt	221.6	28.0
Long-term debt	489.9	417.5
Income taxes currently payable	117.2	83.1
Deferred income taxes	1,030.2	1,022.5
Separate account liabilities	41,957.0	45,744.4
Other liabilities	5,958.8	5,845.5
Total liabilities	57,156.6	59,537.1
Redeemable noncontrolling interest	78.0	58.0
Stockholders' equity
Additional paid-in capital	8,984.8	8,964.2
Retained earnings (accumulated deficit)	3,504.1	2,858.6
Accumulated other comprehensive income	(77.8)	545.9
Total stockholders' equity attributable to Principal Financial Group, Inc.	12,411.1	12,368.7
Noncontrolling interest	70.2	52.5
Total stockholders' equity	12,481.3	12,421.2
Total liabilities and stockholders' equity	69,715.9	72,016.3
Eliminations, Notes Guarantor
Assets
Fixed maturities, available-for-sale	(376.6)	(390.5)
Mortgage loans	(469.2)	(349.6)
Investment in unconsolidated entities	(20,562.4)	(20,164.3)
Other investments	(2,731.3)	(2,610.7)
Cash and cash equivalents	(412.5)	(404.8)	$ 137.5	$ 13.6
Accrued investment income	(1.7)	(1.4)
Premiums due and other receivables	(2,444.0)	(2,190.0)
Other assets	(3,349.4)	(3,009.0)
Total assets	(30,347.1)	(29,120.3)
Liabilities
Contractholder funds	(315.9)	(309.1)
Future policy benefits and claims	(503.5)	(449.7)
Other policyholder funds	(1.0)	(0.5)
Long-term debt	(451.7)	(335.2)
Income taxes currently payable	(110.6)	(71.6)
Deferred income taxes	(617.3)	(617.1)
Other liabilities	(7,409.8)	(6,782.5)
Total liabilities	(9,409.8)	(8,565.7)
Stockholders' equity
Common stock	(2.5)	(2.5)
Additional paid-in capital	(14,307.0)	(14,239.2)
Retained earnings (accumulated deficit)	(5,793.8)	(4,675.8)
Accumulated other comprehensive income	(829.3)	(1,632.6)
Total stockholders' equity attributable to Principal Financial Group, Inc.	(20,932.6)	(20,550.1)
Noncontrolling interest	(4.7)	(4.5)
Total stockholders' equity	(20,937.3)	(20,554.6)
Total liabilities and stockholders' equity	$ (30,347.1)	$ (29,120.3)
Series A
Stockholders' equity
Preferred stock, value
Series B
Stockholders' equity
Preferred stock, value		$ 0.1
Series B | Principal Financial Group, Inc. Parent Only
Stockholders' equity
Preferred stock, value		$ 0.1