Condensed Consolidating Financial Information - Shelf Registration Guarantor, Statements of Financial Position (Details) - USD ($) $ in Millions	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Assets
Fixed maturities, available-for-sale	$ 74,269.2		$ 70,106.2
Fixed maturities, trading	583.8		675.9
Equity securities	1,778.5		1,879.4
Mortgage loans	16,696.6		16,486.9
Real estate	1,699.7		1,714.8
Policy loans	796.4		798.0
Investment in unconsolidated entities	724.3		830.2
Other investments	4,010.1		3,860.0
Cash and cash equivalents	3,150.4		2,515.9
Accrued investment income	702.1		686.6
Premiums due and other receivables	1,760.0		1,740.3
Deferred acquisition costs	3,487.7		3,521.3
Property and equipment	993.1		967.7
Goodwill	1,630.9		1,693.8
Other intangibles	1,709.3		1,786.7
Separate account assets	155,936.3		165,468.0
Other assets	1,381.0		1,356.1
Total assets	271,309.4		276,087.8
Liabilities
Contractholder funds	42,308.4		41,367.5
Future policy benefits and claims	42,896.1		40,838.2
Other policyholder funds	987.4		959.4
Short-term debt	76.3		93.4
Long-term debt	4,163.4		3,734.1
Income taxes currently payable	16.3		16.2
Deferred income taxes	2,061.5		1,796.6
Separate account liabilities	155,936.3		165,468.0
Other liabilities	7,280.0		6,863.7
Total liabilities	255,725.7		261,137.1
Redeemable noncontrolling interest	272.7	$ 239.7	264.9	$ 304.5	$ 537.7	$ 391.2
Stockholders' equity
Common stock	4.8		4.8
Additional paid-in capital	10,259.6		10,182.6
Retained earnings	11,441.4		11,074.3
Accumulated other comprehensive income	1,450.7		1,037.9
Treasury stock, at cost	(7,913.4)		(7,681.6)
Total stockholders' equity attributable to Principal Financial Group, Inc.	15,243.1		14,618.0
Noncontrolling interest	67.9		67.8
Total stockholders' equity	15,311.0	$ 12,972.6	14,685.8	$ 14,102.0	$ 12,762.2	$ 11,456.0
Total liabilities and stockholders' equity	271,309.4		276,087.8
Principal Financial Group, Inc. parent only | Legal Entities
Assets
Fixed maturities, available-for-sale	712.1		265.8
Fixed maturities, trading	232.7		268.2
Investment in unconsolidated entities	18,185.5		17,539.6
Other investments	10.4		10.4
Cash and cash equivalents	454.6		394.9
Accrued investment income	2.3		1.6
Other assets	382.8		383.2
Total assets	19,980.4		18,863.7
Liabilities
Long-term debt	4,119.7		3,625.5
Income taxes currently payable	0.7
Other liabilities	616.9		620.2
Total liabilities	4,737.3		4,245.7
Stockholders' equity
Common stock	4.8		4.8
Additional paid-in capital	10,259.6		10,182.6
Retained earnings	11,441.4		11,074.3
Accumulated other comprehensive income	1,450.7		1,037.9
Treasury stock, at cost	(7,913.4)		(7,681.6)
Total stockholders' equity attributable to Principal Financial Group, Inc.	15,243.1		14,618.0
Total stockholders' equity	15,243.1		14,618.0
Total liabilities and stockholders' equity	19,980.4		18,863.7
Principal Financial Services, Inc. only | Legal Entities
Assets
Equity securities	37.3		41.4
Investment in unconsolidated entities	17,277.9		16,664.1
Other investments	166.7		251.9
Cash and cash equivalents	723.6		598.4
Accrued investment income	0.1		0.4
Premiums due and other receivables	103.2		100.0
Goodwill	618.5		618.5
Other intangibles	517.6		531.7
Other assets	28.0		35.2
Total assets	19,472.9		18,841.6
Liabilities
Long-term debt	297.8		320.7
Income taxes currently payable	3.3
Deferred income taxes	15.8		5.9
Other liabilities	1,113.9		1,113.6
Total liabilities	1,430.8		1,440.2
Stockholders' equity
Additional paid-in capital	9,697.2		9,658.3
Retained earnings	6,477.8		6,263.5
Accumulated other comprehensive income	1,867.1		1,479.6
Total stockholders' equity attributable to Principal Financial Group, Inc.	18,042.1		17,401.4
Total stockholders' equity	18,042.1		17,401.4
Total liabilities and stockholders' equity	19,472.9		18,841.6
Principal Life Insurance Company and other subsidiaries combined | Legal Entities
Assets
Fixed maturities, available-for-sale	73,557.1		69,840.4
Fixed maturities, trading	351.1		407.7
Equity securities	1,741.2		1,838.0
Mortgage loans	16,696.6		16,486.9
Real estate	1,699.7		1,714.8
Policy loans	796.4		798.0
Investment in unconsolidated entities	532.3		649.4
Other investments	3,833.0		3,597.7
Cash and cash equivalents	3,054.2		2,656.2
Accrued investment income	699.7		684.6
Premiums due and other receivables	1,954.9		1,961.1
Deferred acquisition costs	3,487.7		3,521.3
Property and equipment	993.1		967.7
Goodwill	1,012.4		1,075.3
Other intangibles	1,191.7		1,255.0
Separate account assets	155,936.3		165,468.0
Other assets	1,545.0		1,509.5
Total assets	269,082.4		274,431.6
Liabilities
Contractholder funds	42,308.4		41,367.5
Future policy benefits and claims	42,896.1		40,838.2
Other policyholder funds	987.4		959.4
Short-term debt	76.3		93.4
Long-term debt	43.7		108.7
Income taxes currently payable	60.4		66.4
Deferred income taxes	2,552.9		2,285.8
Separate account liabilities	155,936.3		165,468.0
Other liabilities	6,431.9		6,104.7
Total liabilities	251,293.4		257,292.1
Redeemable noncontrolling interest	272.7		264.9
Stockholders' equity
Common stock	11.0		11.0
Additional paid-in capital	12,341.1		12,157.9
Retained earnings	3,057.8		2,985.1
Accumulated other comprehensive income	2,040.5		1,654.8
Treasury stock, at cost	(2.0)		(2.0)
Total stockholders' equity attributable to Principal Financial Group, Inc.	17,448.4		16,806.8
Noncontrolling interest	67.9		67.8
Total stockholders' equity	17,516.3		16,874.6
Total liabilities and stockholders' equity	269,082.4		274,431.6
Principal Life Insurance Company and other subsidiaries combined | Legal Entities | Nonqualified benefit plans
Stockholders' equity
Assets held in Rabbi trusts	741.1		731.9
Liabilities held in Rabbi trusts	597.5		593.7
Eliminations, Shelf Registration Debt Guarantor | Eliminations
Assets
Investment in unconsolidated entities	(35,271.4)		(34,022.9)
Cash and cash equivalents	(1,082.0)		(1,133.6)
Premiums due and other receivables	(298.1)		(320.8)
Other assets	(574.8)		(571.8)
Total assets	(37,226.3)		(36,049.1)
Liabilities
Long-term debt	(297.8)		(320.8)
Income taxes currently payable	(48.1)		(50.2)
Deferred income taxes	(507.2)		(495.1)
Other liabilities	(882.7)		(974.8)
Total liabilities	(1,735.8)		(1,840.9)
Stockholders' equity
Common stock	(11.0)		(11.0)
Additional paid-in capital	(22,038.3)		(21,816.2)
Retained earnings	(9,535.6)		(9,248.6)
Accumulated other comprehensive income	(3,907.6)		(3,134.4)
Treasury stock, at cost	2.0		2.0
Total stockholders' equity attributable to Principal Financial Group, Inc.	(35,490.5)		(34,208.2)
Total stockholders' equity	(35,490.5)		(34,208.2)
Total liabilities and stockholders' equity	$ (37,226.3)		$ (36,049.1)