Condensed Consolidating Financial Information - Shelf Registration 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 Financial Services, Inc. Only | Legal Entities
Assets
Equity securities	14.0		11.9
Investment in unconsolidated entities	16,318.6		14,325.5
Other investments	161.2		100.9
Cash and cash equivalents	1,603.0		649.0
Accrued investment income	3.0		0.3
Premiums due and other receivables	13.5		0.5
Other assets	61.0		38.7
Total assets	18,174.3		15,126.8
Liabilities
Long-term debt	343.8		366.7
Income taxes currently payable			9.8
Deferred income taxes	21.6		13.9
Other liabilities	1,217.8		1,002.9
Total liabilities	1,583.2		1,393.3
Stockholders' equity
Additional paid-in capital	9,123.0		9,100.0
Retained earnings (accumulated deficit)	6,412.8		5,716.8
Accumulated other comprehensive income (loss)	1,055.3		(1,083.3)
Total stockholders' equity attributable to Principal Financial Group, Inc.	16,591.1		13,733.5
Total stockholders' equity	16,591.1		13,733.5
Total liabilities and stockholders' equity	18,174.3		15,126.8
Principal Life Insurance Company and Other Subsidiaries Combined | Legal Entities
Assets
Fixed maturities, available-for-sale	65,061.7		59,897.0
Fixed maturities, trading	389.8		327.2
Equity securities	1,881.2		1,831.8
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	766.7		726.0
Other investments	3,563.9		3,329.9
Cash and cash equivalents	2,365.2		3,096.8
Accrued investment income	659.0		634.5
Premiums due and other receivables	1,880.4		1,779.6
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	1,600.3		1,521.9
Total assets	266,586.1		242,876.5
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	138.1		129.9
Income taxes currently payable	65.0		61.5
Deferred income taxes	2,154.0		1,446.4
Separate account liabilities	162,848.6		144,987.9
Other liabilities	5,277.6		4,838.1
Total liabilities	249,694.6		227,759.6
Redeemable noncontrolling interest	304.5		391.2
Stockholders' equity
Common stock	11.0		11.0
Additional paid-in capital	11,927.8		12,004.5
Retained earnings (accumulated deficit)	3,343.3		3,549.8
Accumulated other comprehensive income (loss)	1,239.6		(903.6)
Treasury stock, at cost	(2.0)		(2.0)
Total stockholders' equity attributable to Principal Financial Group, Inc.	16,519.7		14,659.7
Noncontrolling interest	67.3		66.0
Total stockholders' equity	16,587.0		14,725.7
Total liabilities and stockholders' equity	266,586.1		242,876.5
Principal Life Insurance Company 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, Shelf Registration Debt Guarantor | Eliminations
Assets
Investment in unconsolidated entities	(32,877.3)		(28,045.1)
Cash and cash equivalents	(1,268.1)		(1,103.2)
Premiums due and other receivables	(344.7)		(367.0)
Other assets	(560.5)		(556.7)
Total assets	(35,050.6)		(30,072.0)
Liabilities
Long-term debt	(343.7)		(366.8)
Income taxes currently payable	(47.2)		(46.0)
Deferred income taxes	(495.2)		(501.9)
Other liabilities	(1,053.7)		(764.1)
Total liabilities	(1,939.8)		(1,678.8)
Stockholders' equity
Common stock	(11.0)		(11.0)
Additional paid-in capital	(21,050.8)		(21,104.5)
Retained earnings (accumulated deficit)	(9,756.1)		(9,266.6)
Accumulated other comprehensive income (loss)	(2,294.9)		1,986.9
Treasury stock, at cost	2.0		2.0
Total stockholders' equity attributable to Principal Financial Group, Inc.	(33,110.8)		(28,393.2)
Total stockholders' equity	(33,110.8)		(28,393.2)
Total liabilities and stockholders' equity	$ (35,050.6)		$ (30,072.0)