Condensed Consolidating Financial Information - Shelf Registration 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 Financial Services, Inc. only | Legal Entities
Assets
Equity securities	41.4	11.9
Investment in unconsolidated entities	16,664.1	14,325.5
Other investments	251.9	100.9
Cash and cash equivalents	598.4	649.0
Accrued investment income	0.4	0.3
Premiums due and other receivables	100.0	0.5
Goodwill	618.5
Other intangibles	531.7
Other assets	35.2	38.7
Total assets	18,841.6	15,126.8
Liabilities
Long-term debt	320.7	366.7
Income taxes currently payable		9.8
Deferred income taxes	5.9	13.9
Other liabilities	1,113.6	1,002.9
Total liabilities	1,440.2	1,393.3
Stockholders' equity
Additional paid-in capital	9,658.3	9,100.0
Retained earnings (accumulated deficit)	6,263.5	5,716.8
Accumulated other comprehensive income (loss)	1,479.6	(1,083.3)
Total stockholders' equity attributable to Principal Financial Group, Inc.	17,401.4	13,733.5
Total stockholders' equity	17,401.4	13,733.5
Total liabilities and stockholders' equity	18,841.6	15,126.8
Principal Life Insurance Company and other subsidiaries combined | Legal Entities
Assets
Fixed maturities, available-for-sale	69,840.4	59,897.0
Fixed maturities, trading	407.7	327.2
Equity securities	1,838.0	1,831.8
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	649.4	726.0
Other investments	3,597.7	3,329.9
Cash and cash equivalents	2,656.2	3,096.8
Accrued investment income	684.6	634.5
Premiums due and other receivables	1,961.1	1,779.6
Deferred acquisition costs	3,521.3	3,693.5
Property and equipment	967.7	767.3
Goodwill	1,075.3	1,100.0
Other intangibles	1,255.0	1,315.1
Separate account assets	165,468.0	144,987.9
Other assets	1,509.5	1,521.9
Total assets	274,431.6	242,876.5
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	108.7	129.9
Income taxes currently payable	66.4	61.5
Deferred income taxes	2,285.8	1,446.4
Separate account liabilities	165,468.0	144,987.9
Other liabilities	6,104.7	4,838.1
Total liabilities	257,292.1	227,759.6
Redeemable noncontrolling interest	264.9	391.2
Stockholders' equity
Common stock	11.0	11.0
Additional paid-in capital	12,157.9	12,004.5
Retained earnings (accumulated deficit)	2,985.1	3,549.8
Accumulated other comprehensive income (loss)	1,654.8	(903.6)
Treasury stock, at cost	(2.0)	(2.0)
Total stockholders' equity attributable to Principal Financial Group, Inc.	16,806.8	14,659.7
Noncontrolling interest	67.8	66.0
Total stockholders' equity	16,874.6	14,725.7
Total liabilities and stockholders' equity	274,431.6	242,876.5
Principal Life Insurance Company 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, Shelf Registration Debt Guarantor | Eliminations
Assets
Investment in unconsolidated entities	(34,022.9)	(28,045.1)
Cash and cash equivalents	(1,133.6)	(1,103.2)
Premiums due and other receivables	(320.8)	(367.0)
Other assets	(571.8)	(556.7)
Total assets	(36,049.1)	(30,072.0)
Liabilities
Long-term debt	(320.8)	(366.8)
Income taxes currently payable	(50.2)	(46.0)
Deferred income taxes	(495.1)	(501.9)
Other liabilities	(974.8)	(764.1)
Total liabilities	(1,840.9)	(1,678.8)
Stockholders' equity
Common stock	(11.0)	(11.0)
Additional paid-in capital	(21,816.2)	(21,104.5)
Retained earnings (accumulated deficit)	(9,248.6)	(9,266.6)
Accumulated other comprehensive income (loss)	(3,134.4)	1,986.9
Treasury stock, at cost	2.0	2.0
Total stockholders' equity attributable to Principal Financial Group, Inc.	(34,208.2)	(28,393.2)
Total stockholders' equity	(34,208.2)	(28,393.2)
Total liabilities and stockholders' equity	$ (36,049.1)	$ (30,072.0)