Condensed Consolidating Financial Information - Shelf Registration Guarantor, Statements of Financial Position (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Assets
Fixed maturities, available-for-sale	$ 68,707.8	$ 70,106.2
Fixed maturities, trading	596.0	675.9
Equity securities	1,664.1	1,879.4
Mortgage loans	16,645.9	16,486.9
Real estate	1,657.5	1,714.8
Policy loans	797.0	798.0
Investment in unconsolidated entities	728.6	830.2
Other investments	3,978.4	3,860.0
Cash and cash equivalents	3,482.7	2,515.9
Accrued investment income	721.7	686.6
Premiums due and other receivables	1,691.0	1,740.3
Deferred acquisition costs	3,679.8	3,521.3
Property and equipment	973.8	967.7
Goodwill	1,616.4	1,693.8
Other intangibles	1,716.6	1,786.7
Separate account assets	136,836.0	165,468.0
Other assets	1,410.6	1,356.1
Total assets	246,903.9	276,087.8
Liabilities
Contractholder funds	42,126.6	41,367.5
Future policy benefits and claims	41,244.0	40,838.2
Other policyholder funds	860.9	959.4
Short-term debt	105.6	93.4
Long-term debt	3,670.1	3,734.1
Income taxes currently payable	14.7	16.2
Deferred income taxes	1,450.3	1,796.6
Separate account liabilities (2020 and 2019 include $30,858.6 million and $39,130.3 million related to consolidated variable interest entities)	136,836.0	165,468.0
Other liabilities	7,383.4	6,863.7
Total liabilities	233,691.6	261,137.1
Redeemable noncontrolling interest	239.7	264.9	$ 537.7	$ 391.2
Stockholders' equity
Common stock	4.8	4.8
Additional paid-in capital	10,216.5	10,182.6
Retained earnings (accumulated deficit)	11,198.8	11,074.3
Accumulated other comprehensive income	(599.2)	1,037.9
Treasury stock, at cost	(7,913.4)	(7,681.6)
Total stockholders' equity attributable to Principal Financial Group, Inc.	12,907.5	14,618.0
Noncontrolling interest	65.1	67.8
Total stockholders' equity	12,972.6	14,685.8	$ 12,762.2	$ 11,456.0
Total liabilities and stockholders' equity	246,903.9	276,087.8
Principal Financial Group, Inc. parent only | Legal Entities
Assets
Fixed maturities, available-for-sale	290.5	265.8
Fixed maturities, trading	258.5	268.2
Investment in unconsolidated entities	15,786.7	17,539.6
Other investments	10.4	10.4
Cash and cash equivalents	453.3	394.9
Accrued investment income	1.5	1.6
Other assets	391.1	383.2
Total assets	17,192.0	18,863.7
Liabilities
Long-term debt	3,626.0	3,625.5
Income taxes currently payable	12.6
Other liabilities	645.9	620.2
Total liabilities	4,284.5	4,245.7
Stockholders' equity
Common stock	4.8	4.8
Additional paid-in capital	10,216.5	10,182.6
Retained earnings (accumulated deficit)	11,198.8	11,074.3
Accumulated other comprehensive income	(599.2)	1,037.9
Treasury stock, at cost	(7,913.4)	(7,681.6)
Total stockholders' equity attributable to Principal Financial Group, Inc.	12,907.5	14,618.0
Total stockholders' equity	12,907.5	14,618.0
Total liabilities and stockholders' equity	17,192.0	18,863.7
Principal Financial Services, Inc. only | Legal Entities
Assets
Equity securities	32.7	41.4
Investment in unconsolidated entities	14,883.5	16,664.1
Other investments	198.2	251.9
Cash and cash equivalents	663.6	598.4
Accrued investment income	0.1	0.4
Premiums due and other receivables	110.4	100.0
Goodwill	618.5	618.5
Other intangibles	524.6	531.7
Other assets	47.1	35.2
Total assets	17,078.7	18,841.6
Liabilities
Long-term debt	313.9	320.7
Deferred income taxes	19.5	5.9
Other liabilities	1,098.9	1,113.6
Total liabilities	1,432.3	1,440.2
Stockholders' equity
Additional paid-in capital	9,668.8	9,658.3
Retained earnings (accumulated deficit)	6,149.5	6,263.5
Accumulated other comprehensive income	(171.9)	1,479.6
Total stockholders' equity attributable to Principal Financial Group, Inc.	15,646.4	17,401.4
Total stockholders' equity	15,646.4	17,401.4
Total liabilities and stockholders' equity	17,078.7	18,841.6
Principal Life Insurance Company and other subsidiaries combined | Legal Entities
Assets
Fixed maturities, available-for-sale	68,417.3	69,840.4
Fixed maturities, trading	337.5	407.7
Equity securities	1,631.4	1,838.0
Mortgage loans	16,645.9	16,486.9
Real estate	1,657.5	1,714.8
Policy loans	797.0	798.0
Investment in unconsolidated entities	541.8	649.4
Other investments	3,769.8	3,597.7
Cash and cash equivalents	3,408.8	2,656.2
Accrued investment income	720.1	684.6
Premiums due and other receivables	1,894.6	1,961.1
Deferred acquisition costs	3,679.8	3,521.3
Property and equipment	973.8	967.7
Goodwill	997.9	1,075.3
Other intangibles	1,192.0	1,255.0
Separate account assets	136,836.0	165,468.0
Other assets	1,547.2	1,509.5
Total assets	245,048.4	274,431.6
Liabilities
Contractholder funds	42,126.6	41,367.5
Future policy benefits and claims	41,244.0	40,838.2
Other policyholder funds	860.9	959.4
Short-term debt	105.6	93.4
Long-term debt	44.1	108.7
Income taxes currently payable	38.0	66.4
Deferred income taxes	1,951.8	2,285.8
Separate account liabilities (2020 and 2019 include $30,858.6 million and $39,130.3 million related to consolidated variable interest entities)	136,836.0	165,468.0
Other liabilities	6,510.4	6,104.7
Total liabilities	229,717.4	257,292.1
Redeemable noncontrolling interest	239.7	264.9
Stockholders' equity
Common stock	11.0	11.0
Additional paid-in capital	12,243.7	12,157.9
Retained earnings (accumulated deficit)	2,788.2	2,985.1
Accumulated other comprehensive income	(14.7)	1,654.8
Treasury stock, at cost	(2.0)	(2.0)
Total stockholders' equity attributable to Principal Financial Group, Inc.	15,026.2	16,806.8
Noncontrolling interest	65.1	67.8
Total stockholders' equity	15,091.3	16,874.6
Total liabilities and stockholders' equity	245,048.4	274,431.6
Principal Life Insurance Company and other subsidiaries combined | Legal Entities | Nonqualified benefit plans
Stockholders' equity
Assets held in Rabbi trusts	720.1	731.9
Liabilities held in Rabbi trusts	579.8	593.7
Eliminations, Shelf Registration Debt Guarantor | Eliminations
Assets
Investment in unconsolidated entities	(30,483.4)	(34,022.9)
Cash and cash equivalents	(1,043.0)	(1,133.6)
Premiums due and other receivables	(314.0)	(320.8)
Other assets	(574.8)	(571.8)
Total assets	(32,415.2)	(36,049.1)
Liabilities
Long-term debt	(313.9)	(320.8)
Income taxes currently payable	(35.9)	(50.2)
Deferred income taxes	(521.0)	(495.1)
Other liabilities	(871.8)	(974.8)
Total liabilities	(1,742.6)	(1,840.9)
Stockholders' equity
Common stock	(11.0)	(11.0)
Additional paid-in capital	(21,912.5)	(21,816.2)
Retained earnings (accumulated deficit)	(8,937.7)	(9,248.6)
Accumulated other comprehensive income	186.6	(3,134.4)
Treasury stock, at cost	2.0	2.0
Total stockholders' equity attributable to Principal Financial Group, Inc.	(30,672.6)	(34,208.2)
Total stockholders' equity	(30,672.6)	(34,208.2)
Total liabilities and stockholders' equity	$ (32,415.2)	$ (36,049.1)