Condensed Consolidating Financial Information - Guarantor, Statements of Financial Position (Details) - USD ($) $ in Millions	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Assets
Fixed maturities, available-for-sale	$ 65,268.1		$ 60,108.5
Fixed maturities, trading	686.8		636.1
Equity securities	1,895.2		1,843.7
Mortgage loans	16,136.8		15,336.9
Real estate	1,740.4		1,729.7
Policy loans	800.9		801.4
Investment in unconsolidated entities	931.4		868.8
Other investments	3,735.6		3,441.5
Cash and cash equivalents	3,315.9		2,977.5
Accrued investment income	663.8		636.2
Premiums due and other receivables	1,549.2		1,413.1
Deferred acquisition costs	3,519.5		3,693.5
Property and equipment	959.5		767.3
Goodwill	1,111.5		1,100.0
Other intangibles	1,300.7		1,315.1
Separate account assets	162,848.6		144,987.9
Other assets	1,454.1		1,378.9
Total assets	267,918.0		243,036.1
Liabilities
Contractholder funds	39,658.8		39,699.7
Future policy benefits and claims	38,479.5		35,664.8
Other policyholder funds	996.2		888.4
Short-term debt	76.8		42.9
Long-term debt	3,762.4		3,259.6
Income taxes currently payable	19.8		25.3
Deferred income taxes	1,687.5		958.4
Separate account liabilities	162,848.6		144,987.9
Other liabilities	5,981.9		5,661.9
Total liabilities	253,511.5		231,188.9
Redeemable noncontrolling interest	304.5	$ 537.7	391.2	$ 138.9	$ 127.8	$ 101.3
Stockholders' equity
Common stock	4.8		4.8
Additional paid-in capital	10,130.7		10,060.7
Retained earnings (accumulated deficit)	10,805.8		10,290.2
Accumulated other comprehensive income (loss)	647.7		(1,565.1)
Treasury stock, at cost	(7,554.3)		(7,400.6)
Total stockholders' equity attributable to Principal Financial Group, Inc.	14,034.7		11,390.0
Noncontrolling interest	67.3		66.0
Total stockholders' equity	14,102.0	$ 12,762.2	11,456.0	$ 11,706.5	$ 12,255.8	$ 12,921.9
Total liabilities and stockholders' equity	267,918.0		243,036.1
Principal Financial Group, Inc. Parent Only | Legal Entities
Assets
Fixed maturities, available-for-sale	206.4		211.5
Fixed maturities, trading	297.0		308.9
Investment in unconsolidated entities	16,723.4		13,862.4
Other investments	10.5		10.7
Cash and cash equivalents	615.8		334.9
Accrued investment income	1.8		1.4
Other assets	353.3		375.0
Total assets	18,208.2		15,104.8
Liabilities
Long-term debt	3,624.2		3,129.8
Income taxes currently payable	2.0
Deferred income taxes	7.1
Other liabilities	540.2		585.0
Total liabilities	4,173.5		3,714.8
Stockholders' equity
Common stock	4.8		4.8
Additional paid-in capital	10,130.7		10,060.7
Retained earnings (accumulated deficit)	10,805.8		10,290.2
Accumulated other comprehensive income (loss)	647.7		(1,565.1)
Treasury stock, at cost	(7,554.3)		(7,400.6)
Total stockholders' equity attributable to Principal Financial Group, Inc.	14,034.7		11,390.0
Total stockholders' equity	14,034.7		11,390.0
Total liabilities and stockholders' equity	18,208.2		15,104.8
Principal Life Insurance Company Only | Legal Entities
Assets
Fixed maturities, available-for-sale	57,676.6		53,401.7
Fixed maturities, trading	223.6		165.5
Equity securities	82.8		84.8
Mortgage loans	15,279.1		14,478.0
Real estate	3.3		1.9
Policy loans	748.8		755.9
Investment in unconsolidated entities	1,930.3		2,324.4
Other investments	6,669.6		5,549.1
Cash and cash equivalents	805.0		1,323.9
Accrued investment income	578.7		563.2
Premiums due and other receivables	1,969.9		1,818.1
Deferred acquisition costs	3,506.3		3,680.2
Property and equipment	739.5		661.3
Goodwill	54.3		54.3
Other intangibles	18.7		19.7
Separate account assets	121,368.4		107,343.0
Other assets	1,190.6		1,135.1
Total assets	212,845.5		193,360.1
Liabilities
Contractholder funds	36,703.3		36,861.7
Future policy benefits and claims	32,840.2		30,690.3
Other policyholder funds	821.2		725.7
Deferred income taxes	1,065.4		405.3
Separate account liabilities	121,368.4		107,343.0
Other liabilities	9,260.6		8,503.2
Total liabilities	202,059.1		184,529.2
Stockholders' equity
Common stock	2.5		2.5
Additional paid-in capital	6,321.0		6,331.6
Retained earnings (accumulated deficit)	2,396.3		2,441.2
Accumulated other comprehensive income (loss)	2,066.6		55.6
Total stockholders' equity attributable to Principal Financial Group, Inc.	10,786.4		8,830.9
Total stockholders' equity	10,786.4		8,830.9
Total liabilities and stockholders' equity	212,845.5		193,360.1
Principal Financial Services, Inc. and Other Subsidiaries Combined | Legal Entities
Assets
Fixed maturities, available-for-sale	7,749.6		6,857.9
Fixed maturities, trading	166.2		161.7
Equity securities	1,812.4		1,758.9
Mortgage loans	1,610.9		1,615.5
Real estate	1,737.1		1,727.8
Policy loans	52.1		45.5
Investment in unconsolidated entities	9,074.5		6,853.8
Other investments	2,518.8		2,517.6
Cash and cash equivalents	2,758.5		2,014.8
Accrued investment income	85.6		77.5
Premiums due and other receivables	4,087.1		3,781.3
Deferred acquisition costs	13.2		13.3
Property and equipment	220.0		106.0
Goodwill	1,057.2		1,045.7
Other intangibles	1,282.0		1,295.4
Separate account assets	41,480.2		37,644.9
Other assets	4,839.4		4,418.3
Total assets	80,544.8		71,935.9
Liabilities
Contractholder funds	3,312.1		3,189.2
Future policy benefits and claims	6,892.8		6,128.6
Other policyholder funds	177.7		165.3
Short-term debt	76.8		42.9
Long-term debt	852.1		851.2
Income taxes currently payable	85.5		80.7
Deferred income taxes	1,111.6		1,056.4
Separate account liabilities	41,480.2		37,644.9
Other liabilities	9,593.3		8,586.5
Total liabilities	63,582.1		57,745.7
Redeemable noncontrolling interest	304.5		391.2
Stockholders' equity
Additional paid-in capital	9,500.0		9,461.2
Retained earnings (accumulated deficit)	6,554.6		5,855.0
Accumulated other comprehensive income (loss)	536.3		(1,583.2)
Total stockholders' equity attributable to Principal Financial Group, Inc.	16,590.9		13,733.0
Noncontrolling interest	67.3		66.0
Total stockholders' equity	16,658.2		13,799.0
Total liabilities and stockholders' equity	80,544.8		71,935.9
Principal Financial Services, Inc. and Other Subsidiaries Combined | Legal Entities | Nonqualified benefit plans
Stockholders' equity
Assets held in Rabbi trusts	699.1		671.7
Liabilities held in Rabbi trusts	566.7		542.4
Eliminations, Notes Guarantor | Eliminations
Assets
Fixed maturities, available-for-sale	(364.5)		(362.6)
Mortgage loans	(753.2)		(756.6)
Investment in unconsolidated entities	(26,796.8)		(22,171.8)
Other investments	(5,463.3)		(4,635.9)
Cash and cash equivalents	(863.4)		(696.1)
Accrued investment income	(2.3)		(5.9)
Premiums due and other receivables	(4,507.8)		(4,186.3)
Other assets	(4,929.2)		(4,549.5)
Total assets	(43,680.5)		(37,364.7)
Liabilities
Contractholder funds	(356.6)		(351.2)
Future policy benefits and claims	(1,253.5)		(1,154.1)
Other policyholder funds	(2.7)		(2.6)
Long-term debt	(713.9)		(721.4)
Income taxes currently payable	(67.7)		(55.4)
Deferred income taxes	(496.6)		(503.3)
Other liabilities	(13,412.2)		(12,012.8)
Total liabilities	(16,303.2)		(14,800.8)
Stockholders' equity
Common stock	(2.5)		(2.5)
Additional paid-in capital	(15,821.0)		(15,792.8)
Retained earnings (accumulated deficit)	(8,950.9)		(8,296.2)
Accumulated other comprehensive income (loss)	(2,602.9)		1,527.6
Total stockholders' equity attributable to Principal Financial Group, Inc.	(27,377.3)		(22,563.9)
Total stockholders' equity	(27,377.3)		(22,563.9)
Total liabilities and stockholders' equity	$ (43,680.5)		$ (37,364.7)