Condensed Consolidating Financial Information - Guarantor, Statements of Financial Position (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2015
Assets
Fixed maturities, available-for-sale	$ 57,799.8	$ 54,846.1
Fixed maturities, trading	241.3	398.4
Equity securities, available-for-sale	103.7	98.9
Equity securities, trading	1,586.4	1,413.4
Mortgage loans	13,770.9	13,230.2
Real estate	1,441.8	1,368.8
Policy loans	808.7	823.8
Investment in unconsolidated entities	810.7	773.2
Other investments	2,487.2	2,882.7
Cash and cash equivalents	2,006.4	2,719.6	$ 2,255.4	$ 2,564.8
Accrued investment income	605.4	580.6
Premiums due and other receivables	1,381.4	1,361.9
Deferred acquisition costs	3,452.2	3,380.2
Property and equipment	750.3	699.0
Goodwill	1,033.3	1,020.8
Other intangibles	1,311.3	1,325.3
Separate account assets	149,347.9	139,832.6
Other assets	1,215.2	1,258.8
Total assets	240,153.9	228,014.3
Liabilities
Contractholder funds	38,199.8	37,953.6
Future policy benefits and claims	30,500.4	29,000.7
Other policyholder funds	944.7	890.4
Short-term debt	42.0	51.4
Long-term debt	3,126.8	3,125.7
Income taxes currently payable	16.5	12.9
Deferred income taxes	1,281.8	972.4
Separate account liabilities	149,347.9	139,832.6
Other liabilities	5,360.9	5,783.3
Total liabilities	228,820.8	217,623.0
Redeemable noncontrolling interest	95.1	97.5	337.1	85.7
Stockholders' equity
Common stock	4.7	4.7
Additional paid-in capital	9,845.1	9,686.0
Retained earnings (accumulated deficit)	8,112.2	7,720.4
Accumulated other comprehensive income (loss)	(116.1)	(675.2)
Treasury stock, at cost	(6,677.8)	(6,508.6)
Total stockholders' equity attributable to Principal Financial Group, Inc.	11,168.1	10,227.3
Noncontrolling interest	69.9	66.5
Total stockholders' equity	11,238.0	10,293.8	10,567.7	9,377.4
Total liabilities and stockholders' equity	240,153.9	228,014.3
Principal Financial Group, Inc. Parent Only
Assets
Investment in unconsolidated entities	13,636.9	12,597.9
Other investments	9.8	9.8
Cash and cash equivalents	778.5	882.6	692.3	578.7
Other assets	525.1	573.7
Total assets	14,950.3	14,064.0
Liabilities
Long-term debt	3,127.3	3,126.4
Other liabilities	654.9	710.3
Total liabilities	3,782.2	3,836.7
Stockholders' equity
Common stock	4.7	4.7
Additional paid-in capital	9,845.1	9,686.0
Retained earnings (accumulated deficit)	8,112.2	7,720.4
Accumulated other comprehensive income (loss)	(116.1)	(675.2)
Treasury stock, at cost	(6,677.8)	(6,508.6)
Total stockholders' equity attributable to Principal Financial Group, Inc.	11,168.1	10,227.3
Total stockholders' equity	11,168.1	10,227.3
Total liabilities and stockholders' equity	14,950.3	14,064.0
Principal Life Insurance Company Only
Assets
Fixed maturities, available-for-sale	51,379.4	48,672.1
Fixed maturities, trading	69.1	135.6
Equity securities, available-for-sale	92.2	96.3
Equity securities, trading	5.0	8.2
Mortgage loans	12,976.1	12,460.7
Real estate	4.2	4.4
Policy loans	768.9	784.8
Investment in unconsolidated entities	1,922.4	2,071.1
Other investments	4,645.5	4,740.0
Cash and cash equivalents	506.5	675.1	482.0	1,127.9
Accrued investment income	524.3	513.7
Premiums due and other receivables	1,607.8	1,538.0
Deferred acquisition costs	3,238.1	3,184.2
Property and equipment	646.6	610.4
Goodwill	54.3	54.3
Other intangibles	22.6	23.5
Separate account assets	111,053.3	103,661.9
Other assets	1,005.2	969.5
Total assets	190,521.5	180,203.8
Liabilities
Contractholder funds	35,646.0	35,337.7
Future policy benefits and claims	25,679.2	24,392.6
Other policyholder funds	822.1	780.7
Deferred income taxes	1,086.1	533.6
Separate account liabilities	111,053.3	103,661.9
Other liabilities	7,657.3	7,300.9
Total liabilities	181,944.0	172,007.4
Stockholders' equity
Common stock	2.5	2.5
Additional paid-in capital	6,341.9	5,305.6
Retained earnings (accumulated deficit)	1,051.0	2,139.9
Accumulated other comprehensive income (loss)	1,182.1	748.4
Total stockholders' equity attributable to Principal Financial Group, Inc.	8,577.5	8,196.4
Total stockholders' equity	8,577.5	8,196.4
Total liabilities and stockholders' equity	190,521.5	180,203.8
Principal Financial Services, Inc. and Other Subsidiaries Combined
Assets
Fixed maturities, available-for-sale	6,786.5	6,559.9
Fixed maturities, trading	172.2	262.8
Equity securities, available-for-sale	11.5	2.6
Equity securities, trading	1,581.4	1,405.2
Mortgage loans	1,350.9	1,289.4
Real estate	1,437.6	1,364.4
Policy loans	39.8	39.0
Investment in unconsolidated entities	6,848.6	6,493.7
Other investments	1,876.6	1,783.0
Cash and cash equivalents	1,394.6	2,082.8	1,561.8	1,253.7
Accrued investment income	83.0	74.5
Premiums due and other receivables	3,072.2	2,836.0
Deferred acquisition costs	214.1	196.0
Property and equipment	103.7	88.6
Goodwill	979.0	966.5
Other intangibles	1,288.7	1,301.8
Separate account assets	38,294.6	36,170.7
Other assets	4,224.2	3,507.7
Total assets	69,759.2	66,424.6
Liabilities
Contractholder funds	2,894.0	2,949.2
Future policy benefits and claims	5,644.3	5,312.1
Other policyholder funds	124.6	111.0
Short-term debt	42.0	51.4
Long-term debt	528.8	495.1
Income taxes currently payable	130.1	124.3
Deferred income taxes	1,241.5	1,111.9
Separate account liabilities	38,294.6	36,170.7
Other liabilities	7,148.9	7,425.9
Total liabilities	56,048.8	53,751.6
Redeemable noncontrolling interest	95.1	97.5
Stockholders' equity
Additional paid-in capital	9,030.0	9,010.9
Retained earnings (accumulated deficit)	4,204.7	3,724.3
Accumulated other comprehensive income (loss)	305.6	(230.9)
Total stockholders' equity attributable to Principal Financial Group, Inc.	13,540.3	12,504.3
Noncontrolling interest	75.0	71.2
Total stockholders' equity	13,615.3	12,575.5
Total liabilities and stockholders' equity	69,759.2	66,424.6
Eliminations, Notes Guarantor
Assets
Fixed maturities, available-for-sale	(366.1)	(385.9)
Mortgage loans	(556.1)	(519.9)
Investment in unconsolidated entities	(21,597.2)	(20,389.5)
Other investments	(4,044.7)	(3,650.1)
Cash and cash equivalents	(673.2)	(920.9)	$ (480.7)	$ (395.5)
Accrued investment income	(1.9)	(7.6)
Premiums due and other receivables	(3,298.6)	(3,012.1)
Other assets	(4,539.3)	(3,792.1)
Total assets	(35,077.1)	(32,678.1)
Liabilities
Contractholder funds	(340.2)	(333.3)
Future policy benefits and claims	(823.1)	(704.0)
Other policyholder funds	(2.0)	(1.3)
Long-term debt	(529.3)	(495.8)
Income taxes currently payable	(113.6)	(111.4)
Deferred income taxes	(1,045.8)	(673.1)
Other liabilities	(10,100.2)	(9,653.8)
Total liabilities	(12,954.2)	(11,972.7)
Stockholders' equity
Common stock	(2.5)	(2.5)
Additional paid-in capital	(15,371.9)	(14,316.5)
Retained earnings (accumulated deficit)	(5,255.7)	(5,864.2)
Accumulated other comprehensive income (loss)	(1,487.7)	(517.5)
Total stockholders' equity attributable to Principal Financial Group, Inc.	(22,117.8)	(20,700.7)
Noncontrolling interest	(5.1)	(4.7)
Total stockholders' equity	(22,122.9)	(20,705.4)
Total liabilities and stockholders' equity	$ (35,077.1)	$ (32,678.1)