Condensed Consolidating Financial Information - Shelf Registration 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 Financial Services, Inc. Only
Assets
Equity securities, trading	5.1
Investment in unconsolidated entities	13,958.5	12,532.4
Other investments	109.3	135.9
Cash and cash equivalents	637.7	1,203.4	807.9	730.5
Accrued investment income	3.6	0.1
Premiums due and other receivables	19.4	0.3
Other assets	189.2	185.6
Total assets	14,922.8	14,057.7
Liabilities
Long-term debt	435.6	142.1
Income taxes currently payable	0.1
Other liabilities	946.8	1,411.3
Total liabilities	1,382.5	1,553.4
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
Total stockholders' equity	13,540.3	12,504.3
Total liabilities and stockholders' equity	14,922.8	14,057.7
Principal Life Insurance Company and Other Subsidiaries Combined
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	1,581.3	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	719.4	697.5
Other investments	2,368.1	2,737.0
Cash and cash equivalents	1,837.8	2,114.8	1,989.1	2,413.3
Accrued investment income	601.8	580.5
Premiums due and other receivables	1,797.5	1,503.1
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	1,635.8	1,200.9
Total assets	240,602.9	227,271.3
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	118.9	127.9
Long-term debt	(0.5)	(0.8)
Income taxes currently payable	121.5	68.3
Deferred income taxes	2,315.6	1,619.3
Separate account liabilities	149,347.9	139,832.6
Other liabilities	4,840.0	4,962.1
Total liabilities	226,388.3	214,454.1
Redeemable noncontrolling interest	95.1	97.5
Stockholders' equity
Common stock	17.5	17.8
Additional paid-in capital	11,531.0	10,045.9
Retained earnings (accumulated deficit)	2,320.6	2,940.2
Accumulated other comprehensive income (loss)	182.5	(348.7)
Treasury stock, at cost	(2.0)	(2.0)
Total stockholders' equity attributable to Principal Financial Group, Inc.	14,049.6	12,653.2
Noncontrolling interest	69.9	66.5
Total stockholders' equity	14,119.5	12,719.7
Total liabilities and stockholders' equity	240,602.9	227,271.3
Eliminations, Shelf Registration Debt Guarantor
Assets
Investment in unconsolidated entities	(27,504.1)	(25,054.6)
Cash and cash equivalents	(1,247.6)	(1,481.2)	$ (1,233.9)	$ (1,157.7)
Premiums due and other receivables	(435.5)	(141.5)
Other assets	(1,134.9)	(701.4)
Total assets	(30,322.1)	(27,378.7)
Liabilities
Short-term debt	(76.9)	(76.5)
Long-term debt	(435.6)	(142.0)
Income taxes currently payable	(105.1)	(55.4)
Deferred income taxes	(1,033.8)	(646.9)
Other liabilities	(1,080.8)	(1,300.4)
Total liabilities	(2,732.2)	(2,221.2)
Stockholders' equity
Common stock	(17.5)	(17.8)
Additional paid-in capital	(20,561.0)	(19,056.8)
Retained earnings (accumulated deficit)	(6,525.3)	(6,664.5)
Accumulated other comprehensive income (loss)	(488.1)	579.6
Treasury stock, at cost	2.0	2.0
Total stockholders' equity attributable to Principal Financial Group, Inc.	(27,589.9)	(25,157.5)
Total stockholders' equity	(27,589.9)	(25,157.5)
Total liabilities and stockholders' equity	$ (30,322.1)	$ (27,378.7)