Condensed Consolidating Financial Information - Shelf Registration Guarantor, Statements of Financial Position (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Assets
Fixed maturities, available-for-sale	$ 56,989.1	$ 54,846.1
Fixed maturities, trading	278.0	398.4
Equity securities, available-for-sale	114.0	98.9
Equity securities, trading	1,506.3	1,413.4
Mortgage loans	13,388.1	13,230.2
Real estate	1,433.3	1,368.8
Policy loans	819.6	823.8
Investment in unconsolidated entities	825.8	773.2
Other investments	2,457.8	2,882.7
Cash and cash equivalents	1,534.7	2,719.6	$ 2,142.9	$ 2,564.8
Accrued investment income	625.2	580.6
Premiums due and other receivables	1,447.0	1,361.9
Deferred acquisition costs	3,430.8	3,380.2
Property and equipment	718.8	699.0
Goodwill	1,032.9	1,020.8
Other intangibles	1,322.6	1,325.3
Separate account assets	146,374.7	139,832.6
Other assets	1,197.3	1,258.8
Total assets	235,496.0	228,014.3
Liabilities
Contractholder funds	38,074.1	37,953.6
Future policy benefits and claims	29,690.5	29,000.7
Other policyholder funds	922.7	890.4
Short-term debt	59.5	51.4
Long-term debt	3,126.2	3,125.7
Income taxes currently payable	15.0	12.9
Deferred income taxes	1,048.1	972.4
Separate account liabilities	146,374.7	139,832.6
Other liabilities	5,405.2	5,783.3
Total liabilities	224,716.0	217,623.0
Redeemable noncontrolling interest	95.1	97.5	260.9	85.7
Stockholders' equity
Common stock	4.7	4.7
Additional paid-in capital	9,780.5	9,686.0
Retained earnings (accumulated deficit)	7,937.4	7,720.4
Accumulated other comprehensive income (loss)	(455.1)	(675.2)
Treasury stock, at cost	(6,651.4)	(6,508.6)
Total stockholders' equity attributable to Principal Financial Group, Inc.	10,616.1	10,227.3
Noncontrolling interest	68.8	66.5
Total stockholders' equity	10,684.9	10,293.8	10,109.6	9,377.4
Total liabilities and stockholders' equity	235,496.0	228,014.3
Principal Financial Group, Inc. Parent Only
Assets
Investment in unconsolidated entities	12,951.8	12,597.9
Other investments	9.8	9.8
Cash and cash equivalents	962.6	882.6	734.3	578.7
Other assets	519.4	573.7
Total assets	14,443.6	14,064.0
Liabilities
Long-term debt	3,126.8	3,126.4
Other liabilities	700.7	710.3
Total liabilities	3,827.5	3,836.7
Stockholders' equity
Common stock	4.7	4.7
Additional paid-in capital	9,780.5	9,686.0
Retained earnings (accumulated deficit)	7,937.4	7,720.4
Accumulated other comprehensive income (loss)	(455.1)	(675.2)
Treasury stock, at cost	(6,651.4)	(6,508.6)
Total stockholders' equity attributable to Principal Financial Group, Inc.	10,616.1	10,227.3
Total stockholders' equity	10,616.1	10,227.3
Total liabilities and stockholders' equity	14,443.6	14,064.0
Principal Financial Services, Inc. Only
Assets
Investment in unconsolidated entities	13,007.0	12,532.4
Other investments	143.6	135.9
Cash and cash equivalents	1,170.6	1,203.4	665.0	730.5
Accrued investment income		0.1
Premiums due and other receivables	0.4	0.3
Other assets	188.1	185.6
Total assets	14,509.7	14,057.7
Liabilities
Long-term debt	133.3	142.1
Other liabilities	1,519.9	1,411.3
Total liabilities	1,653.2	1,553.4
Stockholders' equity
Additional paid-in capital	9,008.9	9,010.9
Retained earnings (accumulated deficit)	3,869.5	3,724.3
Accumulated other comprehensive income (loss)	(21.9)	(230.9)
Total stockholders' equity attributable to Principal Financial Group, Inc.	12,856.5	12,504.3
Total stockholders' equity	12,856.5	12,504.3
Total liabilities and stockholders' equity	14,509.7	14,057.7
Principal Life Insurance Company and Other Subsidiaries Combined
Assets
Fixed maturities, available-for-sale	56,989.1	54,846.1
Fixed maturities, trading	278.0	398.4
Equity securities, available-for-sale	114.0	98.9
Equity securities, trading	1,506.3	1,413.4
Mortgage loans	13,388.1	13,230.2
Real estate	1,433.3	1,368.8
Policy loans	819.6	823.8
Investment in unconsolidated entities	739.5	697.5
Other investments	2,304.4	2,737.0
Cash and cash equivalents	1,435.8	2,114.8	1,910.6	2,413.3
Accrued investment income	625.2	580.5
Premiums due and other receivables	1,579.6	1,503.1
Deferred acquisition costs	3,430.8	3,380.2
Property and equipment	718.8	699.0
Goodwill	1,032.9	1,020.8
Other intangibles	1,322.6	1,325.3
Separate account assets	146,374.7	139,832.6
Other assets	1,211.5	1,200.9
Total assets	235,304.2	227,271.3
Liabilities
Contractholder funds	38,074.1	37,953.6
Future policy benefits and claims	29,690.5	29,000.7
Other policyholder funds	922.7	890.4
Short-term debt	136.2	127.9
Long-term debt	(0.6)	(0.8)
Income taxes currently payable	71.7	68.3
Deferred income taxes	1,692.9	1,619.3
Separate account liabilities	146,374.7	139,832.6
Other liabilities	5,068.1	4,962.1
Total liabilities	222,030.3	214,454.1
Redeemable noncontrolling interest	95.1	97.5
Stockholders' equity
Common stock	17.8	17.8
Additional paid-in capital	10,040.2	10,045.9
Retained earnings (accumulated deficit)	3,191.0	2,940.2
Accumulated other comprehensive income (loss)	(137.0)	(348.7)
Treasury stock, at cost	(2.0)	(2.0)
Total stockholders' equity attributable to Principal Financial Group, Inc.	13,110.0	12,653.2
Noncontrolling interest	68.8	66.5
Total stockholders' equity	13,178.8	12,719.7
Total liabilities and stockholders' equity	235,304.2	227,271.3
Eliminations, Shelf Registration Debt Guarantor
Assets
Investment in unconsolidated entities	(25,872.5)	(25,054.6)
Cash and cash equivalents	(2,034.3)	(1,481.2)	$ (1,167.0)	$ (1,157.7)
Premiums due and other receivables	(133.0)	(141.5)
Other assets	(721.7)	(701.4)
Total assets	(28,761.5)	(27,378.7)
Liabilities
Short-term debt	(76.7)	(76.5)
Long-term debt	(133.3)	(142.0)
Income taxes currently payable	(56.7)	(55.4)
Deferred income taxes	(644.8)	(646.9)
Other liabilities	(1,883.5)	(1,300.4)
Total liabilities	(2,795.0)	(2,221.2)
Stockholders' equity
Common stock	(17.8)	(17.8)
Additional paid-in capital	(19,049.1)	(19,056.8)
Retained earnings (accumulated deficit)	(7,060.5)	(6,664.5)
Accumulated other comprehensive income (loss)	158.9	579.6
Treasury stock, at cost	2.0	2.0
Total stockholders' equity attributable to Principal Financial Group, Inc.	(25,966.5)	(25,157.5)
Total stockholders' equity	(25,966.5)	(25,157.5)
Total liabilities and stockholders' equity	$ (28,761.5)	$ (27,378.7)