Condensed Consolidating Financial Information - Guarantor, Statements of Financial Position (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Assets
Fixed maturities, available-for-sale	$ 49,966.5	$ 49,670.8
Fixed maturities, trading	686.8	604.6
Equity securities, available-for-sale	104.5	123.0
Equity securities, trading	1,202.7	840.2
Mortgage loans	12,339.4	11,811.6
Real estate	1,451.8	1,344.6
Policy loans	817.1	829.2
Investment in unconsolidated entities	632.7	815.8
Other investments	2,619.0	2,394.0
Cash and cash equivalents	2,564.8	1,863.9	$ 2,371.8	$ 4,177.2
Accrued investment income	545.6	505.9
Premiums due and other receivables	1,429.3	1,213.0
Deferred acquisition costs	3,276.1	2,993.0	3,077.0	2,590.0
Property and equipment	633.8	590.2
Goodwill	1,009.0	1,007.4	1,100.3
Other intangibles	1,359.2	1,323.5
Separate account assets	136,978.9	140,072.8
Other assets	1,068.7	1,083.5
Total assets	218,685.9	219,087.0
Liabilities
Contractholder funds	35,716.1	34,726.7
Future policy benefits and claims	25,856.5	24,036.6
Other policyholder funds	805.4	812.7
Short-term debt	181.1	28.0
Long-term debt	3,290.8	2,531.2
Income taxes currently payable	18.4	11.5
Deferred income taxes	697.2	1,035.3
Separate account liabilities	136,978.9	140,072.8
Other liabilities	5,678.4	5,542.2
Total liabilities	209,222.8	208,797.0
Redeemable noncontrolling interest	85.7	58.0	247.2	60.4
Stockholders' equity
Common stock	4.7	4.6
Additional paid-in capital	9,544.8	9,945.5
Retained earnings (accumulated deficit)	6,875.9	6,114.1
Accumulated other comprehensive income	(882.5)	50.4
Treasury stock, at cost	(6,231.3)	(5,930.7)
Total stockholders' equity attributable to Principal Financial Group, Inc.	9,311.6	10,184.0
Noncontrolling interest	65.8	48.0
Total stockholders' equity	9,377.4	10,232.0	9,777.0	9,703.4
Total liabilities and stockholders' equity	218,685.9	219,087.0
Principal Financial Group, Inc. Parent Only
Assets
Investment in unconsolidated entities	12,223.4	12,446.5
Other investments	9.7	9.5
Cash and cash equivalents	578.7	412.4	131.5	207.1
Other assets	482.1	450.7
Total assets	13,293.9	13,319.1
Liabilities
Long-term debt	3,247.9	2,448.9
Other liabilities	734.4	686.2
Total liabilities	3,982.3	3,135.1
Stockholders' equity
Common stock	4.7	4.6
Additional paid-in capital	9,544.8	9,945.5
Retained earnings (accumulated deficit)	6,875.9	6,114.1
Accumulated other comprehensive income	(882.5)	50.4
Treasury stock, at cost	(6,231.3)	(5,930.7)
Total stockholders' equity attributable to Principal Financial Group, Inc.	9,311.6	10,184.0
Total stockholders' equity	9,311.6	10,184.0
Total liabilities and stockholders' equity	13,293.9	13,319.1
Principal Life Insurance Company Only
Assets
Fixed maturities, available-for-sale	43,862.7	43,649.7
Fixed maturities, trading	436.2	298.5
Equity securities, available-for-sale	101.7	108.9
Equity securities, trading	0.3	0.3
Mortgage loans	11,696.9	10,972.6
Real estate	6.3	7.0
Policy loans	786.3	799.0
Investment in unconsolidated entities	2,220.5	2,787.9
Other investments	3,944.3	3,416.7
Cash and cash equivalents	1,127.9	602.7	1,332.2	1,698.4
Accrued investment income	477.9	449.2
Premiums due and other receivables	1,512.7	1,237.4
Deferred acquisition costs	3,057.3	2,754.6
Property and equipment	552.0	505.8
Goodwill	54.3	54.3
Other intangibles	24.6	25.7
Separate account assets	94,762.8	94,328.4
Other assets	878.0	873.2
Total assets	165,502.7	162,871.9
Liabilities
Contractholder funds	33,151.7	33,428.3
Future policy benefits and claims	21,914.0	19,768.1
Other policyholder funds	718.1	742.8
Deferred income taxes	415.2	629.9
Separate account liabilities	94,762.8	94,328.4
Other liabilities	6,330.1	5,793.0
Total liabilities	157,291.9	154,690.5
Stockholders' equity
Common stock	2.5	2.5
Additional paid-in capital	5,334.4	5,275.0
Retained earnings (accumulated deficit)	2,232.6	1,817.2
Accumulated other comprehensive income	641.3	1,086.7
Total stockholders' equity attributable to Principal Financial Group, Inc.	8,210.8	8,181.4
Total stockholders' equity	8,210.8	8,181.4
Total liabilities and stockholders' equity	165,502.7	162,871.9
Principal Financial Services, Inc. and Other Subsidiaries Combined
Assets
Fixed maturities, available-for-sale	6,482.5	6,411.6
Fixed maturities, trading	250.6	306.1
Equity securities, available-for-sale	2.8	14.1
Equity securities, trading	1,202.4	839.9
Mortgage loans	1,155.3	1,188.6
Real estate	1,445.5	1,337.6
Policy loans	30.8	30.2
Investment in unconsolidated entities	6,229.8	5,745.7
Other investments	1,636.5	1,578.5
Cash and cash equivalents	1,253.7	1,253.6	894.5	2,286.9
Accrued investment income	76.7	58.1
Premiums due and other receivables	2,465.9	2,165.6
Deferred acquisition costs	218.8	238.4
Property and equipment	81.8	84.4
Goodwill	954.7	953.1
Other intangibles	1,334.6	1,297.8
Separate account assets	42,216.1	45,744.4
Other assets	2,997.1	2,768.6
Total assets	70,035.6	72,016.3
Liabilities
Contractholder funds	2,885.1	1,607.5
Future policy benefits and claims	4,479.3	4,718.2
Other policyholder funds	88.2	70.4
Short-term debt	181.1	28.0
Long-term debt	536.7	417.5
Income taxes currently payable	101.9	83.1
Deferred income taxes	928.9	1,022.5
Separate account liabilities	42,216.1	45,744.4
Other liabilities	6,323.6	5,845.5
Total liabilities	57,740.9	59,537.1
Redeemable noncontrolling interest	85.7	58.0
Stockholders' equity
Additional paid-in capital	9,000.0	8,964.2
Retained earnings (accumulated deficit)	3,522.3	2,858.6
Accumulated other comprehensive income	(383.6)	545.9
Total stockholders' equity attributable to Principal Financial Group, Inc.	12,138.7	12,368.7
Noncontrolling interest	70.3	52.5
Total stockholders' equity	12,209.0	12,421.2
Total liabilities and stockholders' equity	70,035.6	72,016.3
Eliminations, Notes Guarantor
Assets
Fixed maturities, available-for-sale	(378.7)	(390.5)
Mortgage loans	(512.8)	(349.6)
Investment in unconsolidated entities	(20,041.0)	(20,164.3)
Other investments	(2,971.5)	(2,610.7)
Cash and cash equivalents	(395.5)	(404.8)	$ 13.6	$ (15.2)
Accrued investment income	(9.0)	(1.4)
Premiums due and other receivables	(2,549.3)	(2,190.0)
Other assets	(3,288.5)	(3,009.0)
Total assets	(30,146.3)	(29,120.3)
Liabilities
Contractholder funds	(320.7)	(309.1)
Future policy benefits and claims	(536.8)	(449.7)
Other policyholder funds	(0.9)	(0.5)
Long-term debt	(493.8)	(335.2)
Income taxes currently payable	(83.5)	(71.6)
Deferred income taxes	(646.9)	(617.1)
Other liabilities	(7,709.7)	(6,782.5)
Total liabilities	(9,792.3)	(8,565.7)
Stockholders' equity
Common stock	(2.5)	(2.5)
Additional paid-in capital	(14,334.4)	(14,239.2)
Retained earnings (accumulated deficit)	(5,754.9)	(4,675.8)
Accumulated other comprehensive income	(257.7)	(1,632.6)
Total stockholders' equity attributable to Principal Financial Group, Inc.	(20,349.5)	(20,550.1)
Noncontrolling interest	(4.5)	(4.5)
Total stockholders' equity	(20,354.0)	(20,554.6)
Total liabilities and stockholders' equity	$ (30,146.3)	$ (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