Condensed Consolidating Financial Information - 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 Life Insurance Company Only
Assets
Fixed maturities, available-for-sale	50,606.8	48,672.1
Fixed maturities, trading	101.0	135.6
Equity securities, available-for-sale	89.5	96.3
Equity securities, trading	5.2	8.2
Mortgage loans	12,604.9	12,460.7
Real estate	4.4	4.4
Policy loans	779.4	784.8
Investment in unconsolidated entities	2,003.4	2,071.1
Other investments	4,348.6	4,740.0
Cash and cash equivalents	330.5	675.1	692.7	1,127.9
Accrued investment income	548.2	513.7
Premiums due and other receivables	1,619.6	1,538.0
Deferred acquisition costs	3,222.3	3,184.2
Property and equipment	631.5	610.4
Goodwill	54.3	54.3
Other intangibles	23.1	23.5
Separate account assets	108,065.0	103,661.9
Other assets	971.1	969.5
Total assets	186,008.8	180,203.8
Liabilities
Contractholder funds	35,597.1	35,337.7
Future policy benefits and claims	24,931.0	24,392.6
Other policyholder funds	806.3	780.7
Deferred income taxes	455.9	533.6
Separate account liabilities	108,065.0	103,661.9
Other liabilities	7,750.4	7,300.9
Total liabilities	177,605.7	172,007.4
Stockholders' equity
Common stock	2.5	2.5
Additional paid-in capital	5,297.1	5,305.6
Retained earnings (accumulated deficit)	2,241.1	2,139.9
Accumulated other comprehensive income (loss)	862.4	748.4
Total stockholders' equity attributable to Principal Financial Group, Inc.	8,403.1	8,196.4
Total stockholders' equity	8,403.1	8,196.4
Total liabilities and stockholders' equity	186,008.8	180,203.8
Principal Financial Services, Inc. and Other Subsidiaries Combined
Assets
Fixed maturities, available-for-sale	6,748.6	6,559.9
Fixed maturities, trading	177.0	262.8
Equity securities, available-for-sale	24.5	2.6
Equity securities, trading	1,501.1	1,405.2
Mortgage loans	1,338.6	1,289.4
Real estate	1,428.9	1,364.4
Policy loans	40.2	39.0
Investment in unconsolidated entities	6,544.7	6,493.7
Other investments	1,897.1	1,783.0
Cash and cash equivalents	1,783.7	2,082.8	1,128.0	1,253.7
Accrued investment income	79.0	74.5
Premiums due and other receivables	3,014.8	2,836.0
Deferred acquisition costs	208.5	196.0
Property and equipment	87.3	88.6
Goodwill	978.6	966.5
Other intangibles	1,299.5	1,301.8
Separate account assets	38,309.7	36,170.7
Other assets	3,667.5	3,507.7
Total assets	69,129.3	66,424.6
Liabilities
Contractholder funds	2,813.8	2,949.2
Future policy benefits and claims	5,546.6	5,312.1
Other policyholder funds	118.4	111.0
Short-term debt	59.5	51.4
Long-term debt	530.6	495.1
Income taxes currently payable	94.2	124.3
Deferred income taxes	1,256.7	1,111.9
Separate account liabilities	38,309.7	36,170.7
Other liabilities	7,374.6	7,425.9
Total liabilities	56,104.1	53,751.6
Redeemable noncontrolling interest	95.1	97.5
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
Noncontrolling interest	73.6	71.2
Total stockholders' equity	12,930.1	12,575.5
Total liabilities and stockholders' equity	69,129.3	66,424.6
Eliminations, Notes Guarantor
Assets
Fixed maturities, available-for-sale	(366.3)	(385.9)
Mortgage loans	(555.4)	(519.9)
Investment in unconsolidated entities	(20,674.1)	(20,389.5)
Other investments	(3,797.7)	(3,650.1)
Cash and cash equivalents	(1,542.1)	(920.9)	$ (412.1)	$ (395.5)
Accrued investment income	(2.0)	(7.6)
Premiums due and other receivables	(3,187.4)	(3,012.1)
Other assets	(3,960.7)	(3,792.1)
Total assets	(34,085.7)	(32,678.1)
Liabilities
Contractholder funds	(336.8)	(333.3)
Future policy benefits and claims	(787.1)	(704.0)
Other policyholder funds	(2.0)	(1.3)
Long-term debt	(531.2)	(495.8)
Income taxes currently payable	(79.2)	(111.4)
Deferred income taxes	(664.5)	(673.1)
Other liabilities	(10,420.5)	(9,653.8)
Total liabilities	(12,821.3)	(11,972.7)
Stockholders' equity
Common stock	(2.5)	(2.5)
Additional paid-in capital	(14,306.0)	(14,316.5)
Retained earnings (accumulated deficit)	(6,110.6)	(5,864.2)
Accumulated other comprehensive income (loss)	(840.5)	(517.5)
Total stockholders' equity attributable to Principal Financial Group, Inc.	(21,259.6)	(20,700.7)
Noncontrolling interest	(4.8)	(4.7)
Total stockholders' equity	(21,264.4)	(20,705.4)
Total liabilities and stockholders' equity	$ (34,085.7)	$ (32,678.1)