Condensed Consolidating Financial Information - 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 Life Insurance Company only | Legal Entities
Assets
Fixed maturities, available-for-sale	65,862.2		62,363.1
Fixed maturities, trading	233.2		237.5
Equity securities	65.2		69.9
Mortgage loans	15,434.1		15,388.5
Real estate	3.1		3.3
Policy loans	742.5		742.2
Investment in unconsolidated entities	2,657.1		5,036.2
Other investments	6,794.9		3,682.5
Cash and cash equivalents	1,669.6		1,178.1
Accrued investment income	618.2		608.5
Premiums due and other receivables	2,213.4		2,082.3
Deferred acquisition costs	3,477.4		3,509.7
Property and equipment	773.6		733.9
Goodwill	54.3		54.3
Other intangibles	16.9		17.7
Separate account assets	119,347.2		125,801.7
Other assets	1,135.3		1,118.6
Total assets	221,098.2		222,628.0
Liabilities
Contractholder funds	39,235.0		38,334.6
Future policy benefits and claims	37,625.7		35,476.6
Other policyholder funds	805.1		790.3
Deferred income taxes	1,203.4		325.8
Separate account liabilities	119,347.2		125,801.7
Other liabilities	10,684.7		10,533.5
Total liabilities	208,901.1		211,262.5
Stockholders' equity
Common stock	2.5		2.5
Additional paid-in capital	6,332.0		6,331.5
Retained earnings	2,590.9		2,410.8
Accumulated other comprehensive income	3,271.7		2,620.7
Total stockholders' equity attributable to Principal Financial Group, Inc.	12,197.1		11,365.5
Total stockholders' equity	12,197.1		11,365.5
Total liabilities and stockholders' equity	221,098.2		222,628.0
Principal Financial Services, Inc. and other subsidiaries combined | Legal Entities
Assets
Fixed maturities, available-for-sale	8,056.6		7,833.1
Fixed maturities, trading	117.9		170.2
Equity securities	1,713.3		1,809.5
Mortgage loans	1,956.2		1,793.5
Real estate	1,696.6		1,711.5
Policy loans	53.9		55.8
Investment in unconsolidated entities	9,520.6		6,429.5
Other investments	1,983.3		2,859.6
Cash and cash equivalents	1,930.0		1,767.7
Accrued investment income	83.2		81.4
Premiums due and other receivables	4,767.2		4,525.8
Deferred acquisition costs	10.3		11.6
Property and equipment	219.5		233.8
Goodwill	1,576.6		1,639.5
Other intangibles	1,692.4		1,769.0
Separate account assets	36,589.1		39,666.3
Other assets	5,852.0		5,358.4
Total assets	77,818.7		77,716.2
Liabilities
Contractholder funds	3,435.1		3,388.0
Future policy benefits and claims	6,806.7		6,740.7
Other policyholder funds	185.5		171.9
Short-term debt	76.3		93.4
Long-term debt	695.5		763.1
Income taxes currently payable	84.2		81.7
Deferred income taxes	1,365.3		1,965.9
Separate account liabilities	36,589.1		39,666.3
Other liabilities	10,198.7		7,111.2
Total liabilities	59,436.4		59,982.2
Redeemable noncontrolling interest	272.7		264.9
Stockholders' equity
Additional paid-in capital	10,058.5		10,038.7
Retained earnings	6,633.0		6,414.3
Accumulated other comprehensive income	1,350.2		948.3
Total stockholders' equity attributable to Principal Financial Group, Inc.	18,041.7		17,401.3
Noncontrolling interest	67.9		67.8
Total stockholders' equity	18,109.6		17,469.1
Total liabilities and stockholders' equity	77,818.7		77,716.2
Principal Financial Services, Inc. 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, Notes Guarantor | Eliminations
Assets
Fixed maturities, available-for-sale	(361.7)		(355.8)
Mortgage loans	(693.7)		(695.1)
Investment in unconsolidated entities	(29,638.9)		(28,175.1)
Other investments	(4,778.5)		(2,692.5)
Cash and cash equivalents	(903.8)		(824.8)
Accrued investment income	(1.6)		(4.9)
Premiums due and other receivables	(5,220.6)		(4,867.8)
Other assets	(5,989.1)		(5,504.1)
Total assets	(47,587.9)		(43,120.1)
Liabilities
Contractholder funds	(361.7)		(355.1)
Future policy benefits and claims	(1,536.3)		(1,379.1)
Other policyholder funds	(3.2)		(2.8)
Long-term debt	(651.8)		(654.5)
Income taxes currently payable	(68.6)		(65.5)
Deferred income taxes	(507.2)		(495.1)
Other liabilities	(14,220.3)		(11,401.2)
Total liabilities	(17,349.1)		(14,353.3)
Stockholders' equity
Common stock	(2.5)		(2.5)
Additional paid-in capital	(16,390.5)		(16,370.2)
Retained earnings	(9,223.9)		(8,825.1)
Accumulated other comprehensive income	(4,621.9)		(3,569.0)
Total stockholders' equity attributable to Principal Financial Group, Inc.	(30,238.8)		(28,766.8)
Total stockholders' equity	(30,238.8)		(28,766.8)
Total liabilities and stockholders' equity	$ (47,587.9)		$ (43,120.1)