Condensed Consolidating Financial Information (Details 4) - 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 Financial Services, Inc. Only
Assets
Equity securities, trading	4.2
Investment in unconsolidated entities	12,498.9	12,042.5
Other investments	176.3	128.2
Cash and cash equivalents	577.2	907.2	413.0	688.7
Accrued investment income	0.1
Premiums due and other receivables	13.1	0.3
Other assets	232.1	167.6
Total assets	13,501.9	13,245.8
Liabilities
Long-term debt	156.0
Income taxes currently payable		4.3
Deferred income taxes	1.0
Other liabilities	933.8	872.8
Total liabilities	1,090.8	877.1
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
Total stockholders' equity	12,411.1	12,368.7
Total liabilities and stockholders' equity	13,501.9	13,245.8
Principal Life Insurance Company and Other Subsidiaries Combined
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	864.9	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	546.2	781.9
Other investments	2,494.0	2,256.3
Cash and cash equivalents	2,450.0	1,598.0	1,196.4	2,384.0
Accrued investment income	566.7	505.9
Premiums due and other receivables	1,544.5	1,211.8
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	1,091.8	1,117.7
Total assets	215,190.6	218,682.5
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	367.4	182.5
Long-term debt	38.2	82.3
Income taxes currently payable	83.3	54.1
Deferred income taxes	1,471.5	1,626.1
Separate account liabilities	134,045.8	140,072.8
Other liabilities	5,002.7	4,862.4
Total liabilities	202,465.3	206,456.2
Redeemable noncontrolling interest	78.0	58.0
Stockholders' equity
Common stock	17.8	17.8
Additional paid-in capital	9,847.5	9,341.0
Retained earnings (accumulated deficit)	2,930.6	2,283.2
Accumulated other comprehensive income	(212.1)	480.3
Treasury stock, at cost	(2.0)	(2.0)
Total stockholders' equity attributable to Principal Financial Group, Inc.	12,581.8	12,120.3
Noncontrolling interest	65.5	48.0
Total stockholders' equity	12,647.3	12,168.3
Total liabilities and stockholders' equity	215,190.6	218,682.5
Eliminations, Shelf Registration Debt Guarantor
Assets
Investment in unconsolidated entities	(24,948.7)	(24,455.1)
Cash and cash equivalents	(1,091.1)	(1,053.7)	$ (489.3)	$ (832.4)
Premiums due and other receivables	(156.0)	0.9
Other assets	(665.8)	(652.5)
Total assets	(26,861.6)	(26,160.4)
Liabilities
Short-term debt	(145.8)	(154.5)
Long-term debt	(156.0)
Income taxes currently payable	(66.6)	(46.9)
Deferred income taxes	(593.7)	(590.8)
Other liabilities	(906.6)	(879.2)
Total liabilities	(1,868.7)	(1,671.4)
Stockholders' equity
Common stock	(17.8)	(17.8)
Additional paid-in capital	(18,832.3)	(18,305.2)
Retained earnings (accumulated deficit)	(6,434.7)	(5,141.8)
Accumulated other comprehensive income	289.9	(1,026.2)
Treasury stock, at cost	2.0	2.0
Total stockholders' equity attributable to Principal Financial Group, Inc.	(24,992.9)	(24,489.0)
Total stockholders' equity	(24,992.9)	(24,489.0)
Total liabilities and stockholders' equity	$ (26,861.6)	$ (26,160.4)
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