Condensed Consolidating Financial Information - Guarantor, Statements of Financial Position (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Assets
Fixed maturities, available-for-sale	$ 59,137.8	$ 54,846.1
Fixed maturities, trading	232.5	398.4
Equity securities, available-for-sale	118.3	98.9
Equity securities, trading	1,670.9	1,413.4
Mortgage loans	14,020.4	13,230.2
Real estate	1,650.2	1,368.8
Policy loans	812.0	823.8
Investment in unconsolidated entities	850.6	773.2
Other investments	2,710.7	2,882.7
Cash and cash equivalents	2,795.0	2,719.6	$ 2,218.2	$ 2,564.8
Accrued investment income	634.6	580.6
Premiums due and other receivables	1,516.6	1,361.9
Deferred acquisition costs	3,481.4	3,380.2
Property and equipment	754.6	699.0
Goodwill	1,055.2	1,020.8
Other intangibles	1,314.7	1,325.3
Separate account assets	154,085.9	139,832.6
Other assets	1,092.2	1,258.8
Total assets	247,933.6	228,014.3
Liabilities
Contractholder funds	38,208.6	37,953.6
Future policy benefits and claims	32,129.1	29,000.7
Other policyholder funds	926.2	890.4
Short-term debt	58.8	51.4
Long-term debt	3,175.4	3,125.7
Income taxes currently payable	53.7	12.9
Deferred income taxes	1,515.2	972.4
Separate account liabilities	154,085.9	139,832.6
Other liabilities	5,584.3	5,783.3
Total liabilities	235,737.2	217,623.0
Redeemable noncontrolling interest	129.1	97.5	189.9	85.7
Stockholders' equity
Common stock	4.7	4.7
Additional paid-in capital	9,886.8	9,686.0
Retained earnings (accumulated deficit)	8,784.6	7,720.4
Accumulated other comprehensive income (loss)	47.0	(675.2)
Treasury stock, at cost	(6,726.7)	(6,508.6)
Total stockholders' equity attributable to Principal Financial Group, Inc.	11,996.4	10,227.3
Noncontrolling interest	70.9	66.5
Total stockholders' equity	12,067.3	10,293.8	10,863.8	9,377.4
Total liabilities and stockholders' equity	247,933.6	228,014.3
Principal Financial Group, Inc. Parent Only
Assets
Investment in unconsolidated entities	14,631.1	12,597.9
Other investments	10.4	9.8
Cash and cash equivalents	618.1	882.6	761.7	578.7
Other assets	539.9	573.7
Total assets	15,799.5	14,064.0
Liabilities
Long-term debt	3,127.7	3,126.4
Other liabilities	675.4	710.3
Total liabilities	3,803.1	3,836.7
Stockholders' equity
Common stock	4.7	4.7
Additional paid-in capital	9,886.8	9,686.0
Retained earnings (accumulated deficit)	8,784.6	7,720.4
Accumulated other comprehensive income (loss)	47.0	(675.2)
Treasury stock, at cost	(6,726.7)	(6,508.6)
Total stockholders' equity attributable to Principal Financial Group, Inc.	11,996.4	10,227.3
Total stockholders' equity	11,996.4	10,227.3
Total liabilities and stockholders' equity	15,799.5	14,064.0
Principal Life Insurance Company Only
Assets
Fixed maturities, available-for-sale	52,455.8	48,672.1
Fixed maturities, trading	55.7	135.6
Equity securities, available-for-sale	92.9	96.3
Equity securities, trading	6.3	8.2
Mortgage loans	13,288.9	12,460.7
Real estate	4.2	4.4
Policy loans	770.5	784.8
Investment in unconsolidated entities	2,466.1	2,071.1
Other investments	4,674.9	4,740.0
Cash and cash equivalents	781.3	675.1	398.1	1,127.9
Accrued investment income	561.4	513.7
Premiums due and other receivables	1,678.7	1,538.0
Deferred acquisition costs	3,265.6	3,184.2
Property and equipment	654.2	610.4
Goodwill	54.3	54.3
Other intangibles	22.1	23.5
Separate account assets	113,959.3	103,661.9
Other assets	1,019.3	969.5
Total assets	195,811.5	180,203.8
Liabilities
Contractholder funds	35,562.5	35,337.7
Future policy benefits and claims	27,109.2	24,392.6
Other policyholder funds	802.4	780.7
Deferred income taxes	1,152.0	533.6
Separate account liabilities	113,959.3	103,661.9
Other liabilities	7,915.9	7,300.9
Total liabilities	186,501.3	172,007.4
Stockholders' equity
Common stock	2.5	2.5
Additional paid-in capital	6,340.6	5,305.6
Retained earnings (accumulated deficit)	1,723.2	2,139.9
Accumulated other comprehensive income (loss)	1,243.9	748.4
Total stockholders' equity attributable to Principal Financial Group, Inc.	9,310.2	8,196.4
Total stockholders' equity	9,310.2	8,196.4
Total liabilities and stockholders' equity	195,811.5	180,203.8
Principal Financial Services, Inc. and Other Subsidiaries Combined
Assets
Fixed maturities, available-for-sale	7,049.8	6,559.9
Fixed maturities, trading	176.8	262.8
Equity securities, available-for-sale	25.4	2.6
Equity securities, trading	1,664.6	1,405.2
Mortgage loans	1,420.9	1,289.4
Real estate	1,646.0	1,364.4
Policy loans	41.5	39.0
Investment in unconsolidated entities	7,063.8	6,493.7
Other investments	2,065.0	1,783.0
Cash and cash equivalents	2,160.3	2,082.8	1,723.1	1,253.7
Accrued investment income	75.6	74.5
Premiums due and other receivables	3,288.4	2,836.0
Deferred acquisition costs	215.8	196.0
Property and equipment	100.4	88.6
Goodwill	1,000.9	966.5
Other intangibles	1,292.6	1,301.8
Separate account assets	40,126.6	36,170.7
Other assets	4,349.4	3,507.7
Total assets	73,763.8	66,424.6
Liabilities
Contractholder funds	2,989.8	2,949.2
Future policy benefits and claims	5,899.0	5,312.1
Other policyholder funds	125.5	111.0
Short-term debt	58.8	51.4
Long-term debt	709.5	495.1
Income taxes currently payable	338.9	124.3
Deferred income taxes	1,366.4	1,111.9
Separate account liabilities	40,126.6	36,170.7
Other liabilities	7,411.7	7,425.9
Total liabilities	59,026.2	53,751.6
Redeemable noncontrolling interest	129.1	97.5
Stockholders' equity
Additional paid-in capital	9,037.8	9,010.9
Retained earnings (accumulated deficit)	5,037.1	3,724.3
Accumulated other comprehensive income (loss)	457.6	(230.9)
Total stockholders' equity attributable to Principal Financial Group, Inc.	14,532.5	12,504.3
Noncontrolling interest	76.0	71.2
Total stockholders' equity	14,608.5	12,575.5
Total liabilities and stockholders' equity	73,763.8	66,424.6
Principal Financial Services, Inc. and Other Subsidiaries Combined | Nonqualified benefit plans
Stockholders' equity
Assets held in Rabbi trusts	631.2	570.9
Liabilities held in Rabbi trusts	532.6	477.3
Eliminations, Notes Guarantor
Assets
Fixed maturities, available-for-sale	(367.8)	(385.9)
Mortgage loans	(689.4)	(519.9)
Investment in unconsolidated entities	(23,310.4)	(20,389.5)
Other investments	(4,039.6)	(3,650.1)
Cash and cash equivalents	(764.7)	(920.9)	$ (664.7)	$ (395.5)
Accrued investment income	(2.4)	(7.6)
Premiums due and other receivables	(3,450.5)	(3,012.1)
Other assets	(4,816.4)	(3,792.1)
Total assets	(37,441.2)	(32,678.1)
Liabilities
Contractholder funds	(343.7)	(333.3)
Future policy benefits and claims	(879.1)	(704.0)
Other policyholder funds	(1.7)	(1.3)
Long-term debt	(661.8)	(495.8)
Income taxes currently payable	(285.2)	(111.4)
Deferred income taxes	(1,003.2)	(673.1)
Other liabilities	(10,418.7)	(9,653.8)
Total liabilities	(13,593.4)	(11,972.7)
Stockholders' equity
Common stock	(2.5)	(2.5)
Additional paid-in capital	(15,378.4)	(14,316.5)
Retained earnings (accumulated deficit)	(6,760.3)	(5,864.2)
Accumulated other comprehensive income (loss)	(1,701.5)	(517.5)
Total stockholders' equity attributable to Principal Financial Group, Inc.	(23,842.7)	(20,700.7)
Noncontrolling interest	(5.1)	(4.7)
Total stockholders' equity	(23,847.8)	(20,705.4)
Total liabilities and stockholders' equity	$ (37,441.2)	$ (32,678.1)