Condensed Consolidating Financial Information - Guarantor, Statements of Financial Position (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Assets
Fixed maturities, available-for-sale	$ 70,106.2	$ 60,108.5
Fixed maturities, trading	675.9	636.1
Equity securities	1,879.4	1,843.7
Mortgage loans	16,486.9	15,336.9
Real estate	1,714.8	1,729.7
Policy loans	798.0	801.4
Investment in unconsolidated entities	830.2	868.8
Other investments	3,860.0	3,441.5
Cash and cash equivalents	2,515.9	2,977.5
Accrued investment income	686.6	636.2
Premiums due and other receivables	1,740.3	1,413.1
Deferred acquisition costs	3,521.3	3,693.5	$ 3,540.7	$ 3,380.2
Property and equipment	967.7	767.3
Goodwill	1,693.8	1,100.0	1,068.8
Other intangibles	1,786.7	1,315.1
Separate account assets	165,468.0	144,987.9
Other assets	1,356.1	1,378.9
Total assets	276,087.8	243,036.1
Liabilities
Contractholder funds	41,367.5	39,699.7
Future policy benefits and claims	40,838.2	35,664.8
Other policyholder funds	959.4	888.4
Short-term debt	93.4	42.9
Long-term debt	3,734.1	3,259.6
Income taxes currently payable	16.2	25.3
Deferred income taxes	1,796.6	958.4
Separate account liabilities	165,468.0	144,987.9
Other liabilities	6,863.7	5,661.9
Total liabilities	261,137.1	231,188.9
Redeemable noncontrolling interest	264.9	391.2	101.3	97.5
Stockholders' equity
Common stock	4.8	4.8
Additional paid-in capital	10,182.6	10,060.7
Retained earnings (accumulated deficit)	11,074.3	10,290.2
Accumulated other comprehensive income (loss)	1,037.9	(1,565.1)
Treasury stock, at cost	(7,681.6)	(7,400.6)
Total stockholders' equity attributable to Principal Financial Group, Inc.	14,618.0	11,390.0
Noncontrolling interest	67.8	66.0
Total stockholders' equity	14,685.8	11,456.0	$ 12,921.9	$ 10,293.8
Total liabilities and stockholders' equity	276,087.8	243,036.1
Principal Financial Group, Inc. parent only | Legal Entities
Assets
Fixed maturities, available-for-sale	265.8	211.5
Fixed maturities, trading	268.2	308.9
Investment in unconsolidated entities	17,539.6	13,862.4
Other investments	10.4	10.7
Cash and cash equivalents	394.9	334.9
Accrued investment income	1.6	1.4
Other assets	383.2	375.0
Total assets	18,863.7	15,104.8
Liabilities
Long-term debt	3,625.5	3,129.8
Other liabilities	620.2	585.0
Total liabilities	4,245.7	3,714.8
Stockholders' equity
Common stock	4.8	4.8
Additional paid-in capital	10,182.6	10,060.7
Retained earnings (accumulated deficit)	11,074.3	10,290.2
Accumulated other comprehensive income (loss)	1,037.9	(1,565.1)
Treasury stock, at cost	(7,681.6)	(7,400.6)
Total stockholders' equity attributable to Principal Financial Group, Inc.	14,618.0	11,390.0
Total stockholders' equity	14,618.0	11,390.0
Total liabilities and stockholders' equity	18,863.7	15,104.8
Principal Life Insurance Company only | Legal Entities
Assets
Fixed maturities, available-for-sale	62,363.1	53,401.7
Fixed maturities, trading	237.5	165.5
Equity securities	69.9	84.8
Mortgage loans	15,388.5	14,478.0
Real estate	3.3	1.9
Policy loans	742.2	755.9
Investment in unconsolidated entities	5,036.2	2,324.4
Other investments	3,682.5	5,549.1
Cash and cash equivalents	1,178.1	1,323.9
Accrued investment income	608.5	563.2
Premiums due and other receivables	2,082.3	1,818.1
Deferred acquisition costs	3,509.7	3,680.2
Property and equipment	733.9	661.3
Goodwill	54.3	54.3
Other intangibles	17.7	19.7
Separate account assets	125,801.7	107,343.0
Other assets	1,118.6	1,135.1
Total assets	222,628.0	193,360.1
Liabilities
Contractholder funds	38,334.6	36,861.7
Future policy benefits and claims	35,476.6	30,690.3
Other policyholder funds	790.3	725.7
Deferred income taxes	325.8	405.3
Separate account liabilities	125,801.7	107,343.0
Other liabilities	10,533.5	8,503.2
Total liabilities	211,262.5	184,529.2
Stockholders' equity
Common stock	2.5	2.5
Additional paid-in capital	6,331.5	6,331.6
Retained earnings (accumulated deficit)	2,410.8	2,441.2
Accumulated other comprehensive income (loss)	2,620.7	55.6
Total stockholders' equity attributable to Principal Financial Group, Inc.	11,365.5	8,830.9
Total stockholders' equity	11,365.5	8,830.9
Total liabilities and stockholders' equity	222,628.0	193,360.1
Principal Financial Services, Inc. and other subsidiaries combined | Legal Entities
Assets
Fixed maturities, available-for-sale	7,833.1	6,857.9
Fixed maturities, trading	170.2	161.7
Equity securities	1,809.5	1,758.9
Mortgage loans	1,793.5	1,615.5
Real estate	1,711.5	1,727.8
Policy loans	55.8	45.5
Investment in unconsolidated entities	6,429.5	6,853.8
Other investments	2,859.6	2,517.6
Cash and cash equivalents	1,767.7	2,014.8
Accrued investment income	81.4	77.5
Premiums due and other receivables	4,525.8	3,781.3
Deferred acquisition costs	11.6	13.3
Property and equipment	233.8	106.0
Goodwill	1,639.5	1,045.7
Other intangibles	1,769.0	1,295.4
Separate account assets	39,666.3	37,644.9
Other assets	5,358.4	4,418.3
Total assets	77,716.2	71,935.9
Liabilities
Contractholder funds	3,388.0	3,189.2
Future policy benefits and claims	6,740.7	6,128.6
Other policyholder funds	171.9	165.3
Short-term debt	93.4	42.9
Long-term debt	763.1	851.2
Income taxes currently payable	81.7	80.7
Deferred income taxes	1,965.9	1,056.4
Separate account liabilities	39,666.3	37,644.9
Other liabilities	7,111.2	8,586.5
Total liabilities	59,982.2	57,745.7
Redeemable noncontrolling interest	264.9	391.2
Stockholders' equity
Additional paid-in capital	10,038.7	9,461.2
Retained earnings (accumulated deficit)	6,414.3	5,855.0
Accumulated other comprehensive income (loss)	948.3	(1,583.2)
Total stockholders' equity attributable to Principal Financial Group, Inc.	17,401.3	13,733.0
Noncontrolling interest	67.8	66.0
Total stockholders' equity	17,469.1	13,799.0
Total liabilities and stockholders' equity	77,716.2	71,935.9
Principal Financial Services, Inc. and other subsidiaries combined | Legal Entities | Nonqualified benefit plans
Stockholders' equity
Assets held in Rabbi trusts	731.9	671.7
Liabilities held in Rabbi trusts	593.7	542.4
Eliminations, Notes Guarantor | Eliminations
Assets
Fixed maturities, available-for-sale	(355.8)	(362.6)
Mortgage loans	(695.1)	(756.6)
Investment in unconsolidated entities	(28,175.1)	(22,171.8)
Other investments	(2,692.5)	(4,635.9)
Cash and cash equivalents	(824.8)	(696.1)
Accrued investment income	(4.9)	(5.9)
Premiums due and other receivables	(4,867.8)	(4,186.3)
Other assets	(5,504.1)	(4,549.5)
Total assets	(43,120.1)	(37,364.7)
Liabilities
Contractholder funds	(355.1)	(351.2)
Future policy benefits and claims	(1,379.1)	(1,154.1)
Other policyholder funds	(2.8)	(2.6)
Long-term debt	(654.5)	(721.4)
Income taxes currently payable	(65.5)	(55.4)
Deferred income taxes	(495.1)	(503.3)
Other liabilities	(11,401.2)	(12,012.8)
Total liabilities	(14,353.3)	(14,800.8)
Stockholders' equity
Common stock	(2.5)	(2.5)
Additional paid-in capital	(16,370.2)	(15,792.8)
Retained earnings (accumulated deficit)	(8,825.1)	(8,296.2)
Accumulated other comprehensive income (loss)	(3,569.0)	1,527.6
Total stockholders' equity attributable to Principal Financial Group, Inc.	(28,766.8)	(22,563.9)
Total stockholders' equity	(28,766.8)	(22,563.9)
Total liabilities and stockholders' equity	$ (43,120.1)	$ (37,364.7)