Condensed Consolidating Financial Information - 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 Life Insurance Company only | Legal Entities
Assets
Fixed maturities, available-for-sale	61,408.8	62,363.1
Fixed maturities, trading	220.4	237.5
Equity securities	61.7	69.9
Mortgage loans	15,524.6	15,388.5
Real estate	3.2	3.3
Policy loans	743.0	742.2
Investment in unconsolidated entities	4,850.7	5,036.2
Other investments	4,214.3	3,682.5
Cash and cash equivalents	1,908.3	1,178.1
Accrued investment income	641.9	608.5
Premiums due and other receivables	2,067.3	2,082.3
Deferred acquisition costs	3,669.7	3,509.7
Property and equipment	757.7	733.9
Goodwill	54.3	54.3
Other intangibles	17.3	17.7
Separate account assets	105,530.3	125,801.7
Other assets	1,136.9	1,118.6
Total assets	202,810.4	222,628.0
Liabilities
Contractholder funds	39,090.1	38,334.6
Future policy benefits and claims	36,435.7	35,476.6
Other policyholder funds	694.9	790.3
Deferred income taxes	86.2	325.8
Separate account liabilities (2020 and 2019 include $30,858.6 million and $39,130.3 million related to consolidated variable interest entities)	105,530.3	125,801.7
Other liabilities	10,937.5	10,533.5
Total liabilities	192,774.7	211,262.5
Stockholders' equity
Common stock	2.5	2.5
Additional paid-in capital	6,324.1	6,331.5
Retained earnings (accumulated deficit)	2,372.2	2,410.8
Accumulated other comprehensive income	1,336.9	2,620.7
Total stockholders' equity attributable to Principal Financial Group, Inc.	10,035.7	11,365.5
Total stockholders' equity	10,035.7	11,365.5
Total liabilities and stockholders' equity	202,810.4	222,628.0
Principal Financial Services, Inc. and other subsidiaries combined | Legal Entities
Assets
Fixed maturities, available-for-sale	7,367.5	7,833.1
Fixed maturities, trading	117.1	170.2
Equity securities	1,602.4	1,809.5
Mortgage loans	1,816.1	1,793.5
Real estate	1,654.3	1,711.5
Policy loans	54.0	55.8
Investment in unconsolidated entities	5,183.0	6,429.5
Other investments	2,697.5	2,859.6
Cash and cash equivalents	1,982.7	1,767.7
Accrued investment income	80.3	81.4
Premiums due and other receivables	4,629.2	4,525.8
Deferred acquisition costs	10.1	11.6
Property and equipment	216.1	233.8
Goodwill	1,562.1	1,639.5
Other intangibles	1,699.3	1,769.0
Separate account assets	31,305.7	39,666.3
Other assets	5,373.2	5,358.4
Total assets	67,350.6	77,716.2
Liabilities
Contractholder funds	3,395.3	3,388.0
Future policy benefits and claims	6,223.6	6,740.7
Other policyholder funds	170.0	171.9
Short-term debt	105.6	93.4
Long-term debt	697.2	763.1
Income taxes currently payable	58.2	81.7
Deferred income taxes	1,885.0	1,965.9
Separate account liabilities (2020 and 2019 include $30,858.6 million and $39,130.3 million related to consolidated variable interest entities)	31,305.7	39,666.3
Other liabilities	7,558.9	7,111.2
Total liabilities	51,399.5	59,982.2
Redeemable noncontrolling interest	239.7	264.9
Stockholders' equity
Additional paid-in capital	10,030.0	10,038.7
Retained earnings (accumulated deficit)	6,307.6	6,414.3
Accumulated other comprehensive income	(691.3)	948.3
Total stockholders' equity attributable to Principal Financial Group, Inc.	15,646.3	17,401.3
Noncontrolling interest	65.1	67.8
Total stockholders' equity	15,711.4	17,469.1
Total liabilities and stockholders' equity	67,350.6	77,716.2
Principal Financial Services, Inc. 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, Notes Guarantor | Eliminations
Assets
Fixed maturities, available-for-sale	(359.0)	(355.8)
Mortgage loans	(694.8)	(695.1)
Investment in unconsolidated entities	(25,091.8)	(28,175.1)
Other investments	(2,943.8)	(2,692.5)
Cash and cash equivalents	(861.6)	(824.8)
Accrued investment income	(2.0)	(4.9)
Premiums due and other receivables	(5,005.5)	(4,867.8)
Other assets	(5,490.6)	(5,504.1)
Total assets	(40,449.1)	(43,120.1)
Liabilities
Contractholder funds	(358.8)	(355.1)
Future policy benefits and claims	(1,415.3)	(1,379.1)
Other policyholder funds	(4.0)	(2.8)
Long-term debt	(653.1)	(654.5)
Income taxes currently payable	(56.1)	(65.5)
Deferred income taxes	(520.9)	(495.1)
Other liabilities	(11,758.9)	(11,401.2)
Total liabilities	(14,767.1)	(14,353.3)
Stockholders' equity
Common stock	(2.5)	(2.5)
Additional paid-in capital	(16,354.1)	(16,370.2)
Retained earnings (accumulated deficit)	(8,679.8)	(8,825.1)
Accumulated other comprehensive income	(645.6)	(3,569.0)
Total stockholders' equity attributable to Principal Financial Group, Inc.	(25,682.0)	(28,766.8)
Total stockholders' equity	(25,682.0)	(28,766.8)
Total liabilities and stockholders' equity	$ (40,449.1)	$ (43,120.1)