Condensed Consolidating Financial Information - Shelf Registration Guarantor, Statements of Financial Position (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Assets
Fixed maturities, available-for-sale	$ 78,710.3	$ 70,106.2
Fixed maturities, trading	532.1	675.9
Equity securities	2,013.4	1,879.4
Mortgage loans	17,343.0	16,486.9
Real estate	1,797.3	1,714.8
Policy loans	784.0	798.0
Investment in unconsolidated entities	842.7	830.2
Other investments	4,284.1	3,860.0
Cash and cash equivalents	2,849.8	2,515.9
Accrued investment income	710.6	686.6
Premiums due and other receivables	1,723.8	1,740.3
Deferred acquisition costs	3,409.7	3,521.3	$ 3,693.5	$ 3,540.7
Property and equipment	1,019.0	967.7
Goodwill	1,711.0	1,693.8	1,100.0
Other intangibles	1,723.0	1,786.7
Separate account assets	175,951.4	165,468.0
Other assets	1,222.5	1,356.1
Total assets	296,627.7	276,087.8
Liabilities
Contractholder funds	43,237.7	41,367.5
Future policy benefits and claims	45,207.2	40,838.2
Other policyholder funds	1,059.4	959.4
Short-term debt	84.7	93.4
Long-term debt	4,279.2	3,734.1
Income taxes currently payable	22.3	16.2
Deferred income taxes	2,330.8	1,796.6
Separate account liabilities	175,951.4	165,468.0
Other liabilities	7,582.1	6,863.7
Total liabilities	279,754.8	261,137.1
Redeemable noncontrolling interest	255.6	264.9	391.2	101.3
Stockholders' equity
Common stock	4.8	4.8
Additional paid-in capital	10,321.6	10,182.6
Retained earnings	11,838.0	11,074.3
Accumulated other comprehensive income	2,383.1	1,037.9
Treasury stock, at cost	(7,988.6)	(7,681.6)
Total stockholders' equity attributable to Principal Financial Group, Inc.	16,558.9	14,618.0
Noncontrolling interest	58.4	67.8
Total stockholders' equity	16,617.3	14,685.8	$ 11,456.0	$ 12,921.9
Total liabilities and stockholders' equity	296,627.7	276,087.8
Principal Financial Group, Inc. parent only | Legal Entities
Assets
Fixed maturities, available-for-sale	813.2	265.8
Fixed maturities, trading	178.8	268.2
Investment in unconsolidated entities	19,714.3	17,539.6
Other investments	63.7	10.4
Cash and cash equivalents	302.4	394.9
Accrued investment income	2.3	1.6
Other assets	408.1	383.2
Total assets	21,482.8	18,863.7
Liabilities
Long-term debt	4,223.3	3,625.5
Other liabilities	700.6	620.2
Total liabilities	4,923.9	4,245.7
Stockholders' equity
Common stock	4.8	4.8
Additional paid-in capital	10,321.6	10,182.6
Retained earnings	11,838.0	11,074.3
Accumulated other comprehensive income	2,383.1	1,037.9
Treasury stock, at cost	(7,988.6)	(7,681.6)
Total stockholders' equity attributable to Principal Financial Group, Inc.	16,558.9	14,618.0
Total stockholders' equity	16,558.9	14,618.0
Total liabilities and stockholders' equity	21,482.8	18,863.7
Principal Financial Services, Inc. only | Legal Entities
Assets
Equity securities	42.9	41.4
Investment in unconsolidated entities	18,557.9	16,664.1
Other investments	156.3	251.9
Cash and cash equivalents	819.1	598.4
Accrued investment income	0.2	0.4
Premiums due and other receivables	88.8	100.0
Goodwill	618.5	618.5
Other intangibles	503.6	531.7
Other assets	58.0	35.2
Total assets	20,845.3	18,841.6
Liabilities
Long-term debt	274.9	320.7
Deferred income taxes	31.3	5.9
Other liabilities	973.2	1,113.6
Total liabilities	1,279.4	1,440.2
Stockholders' equity
Additional paid-in capital	9,738.4	9,658.3
Retained earnings	6,980.7	6,263.5
Accumulated other comprehensive income	2,846.8	1,479.6
Total stockholders' equity attributable to Principal Financial Group, Inc.	19,565.9	17,401.4
Total stockholders' equity	19,565.9	17,401.4
Total liabilities and stockholders' equity	20,845.3	18,841.6
Principal Life Insurance Company and other subsidiaries combined | Legal Entities
Assets
Fixed maturities, available-for-sale	77,897.1	69,840.4
Fixed maturities, trading	353.3	407.7
Equity securities	1,970.5	1,838.0
Mortgage loans	17,343.0	16,486.9
Real estate	1,797.3	1,714.8
Policy loans	784.0	798.0
Investment in unconsolidated entities	615.9	649.4
Other investments	4,064.1	3,597.7
Cash and cash equivalents	2,740.7	2,656.2
Accrued investment income	708.1	684.6
Premiums due and other receivables	1,910.2	1,961.1
Deferred acquisition costs	3,409.7	3,521.3
Property and equipment	1,019.0	967.7
Goodwill	1,092.5	1,075.3
Other intangibles	1,219.4	1,255.0
Separate account assets	175,951.4	165,468.0
Other assets	1,362.7	1,509.5
Total assets	294,238.9	274,431.6
Liabilities
Contractholder funds	43,237.7	41,367.5
Future policy benefits and claims	45,207.2	40,838.2
Other policyholder funds	1,059.4	959.4
Short-term debt	84.7	93.4
Long-term debt	55.9	108.7
Income taxes currently payable	79.7	66.4
Deferred income taxes	2,824.4	2,285.8
Separate account liabilities	175,951.4	165,468.0
Other liabilities	6,769.6	6,104.7
Total liabilities	275,270.0	257,292.1
Redeemable noncontrolling interest	255.6	264.9
Stockholders' equity
Common stock	5.5	11.0
Additional paid-in capital	12,357.7	12,157.9
Retained earnings	3,285.9	2,985.1
Accumulated other comprehensive income	3,005.8	1,654.8
Treasury stock, at cost		(2.0)
Total stockholders' equity attributable to Principal Financial Group, Inc.	18,654.9	16,806.8
Noncontrolling interest	58.4	67.8
Total stockholders' equity	18,713.3	16,874.6
Total liabilities and stockholders' equity	294,238.9	274,431.6
Principal Life Insurance Company and other subsidiaries combined | Legal Entities | Nonqualified benefit plans
Stockholders' equity
Assets held in Rabbi trusts	801.7	731.9
Liabilities held in Rabbi trusts	653.3	593.7
Eliminations, Shelf Registration Debt Guarantor | Eliminations
Assets
Investment in unconsolidated entities	(38,045.4)	(34,022.9)
Cash and cash equivalents	(1,012.4)	(1,133.6)
Premiums due and other receivables	(275.2)	(320.8)
Other assets	(606.3)	(571.8)
Total assets	(39,939.3)	(36,049.1)
Liabilities
Long-term debt	(274.9)	(320.8)
Income taxes currently payable	(57.4)	(50.2)
Deferred income taxes	(524.9)	(495.1)
Other liabilities	(861.3)	(974.8)
Total liabilities	(1,718.5)	(1,840.9)
Stockholders' equity
Common stock	(5.5)	(11.0)
Additional paid-in capital	(22,096.1)	(21,816.2)
Retained earnings	(10,266.6)	(9,248.6)
Accumulated other comprehensive income	(5,852.6)	(3,134.4)
Treasury stock, at cost		2.0
Total stockholders' equity attributable to Principal Financial Group, Inc.	(38,220.8)	(34,208.2)
Total stockholders' equity	(38,220.8)	(34,208.2)
Total liabilities and stockholders' equity	$ (39,939.3)	$ (36,049.1)