Condensed Consolidating Financial Information - Shelf Registration Guarantor, Statements of Financial Position (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Assets
Fixed maturities, available-for-sale	$ 59,388.4	$ 54,846.1
Fixed maturities, trading	566.0	398.4
Equity securities, available-for-sale	96.0	98.9
Equity securities, trading	1,770.6	1,413.4
Mortgage loans	14,150.5	13,230.2
Real estate	1,736.7	1,368.8
Policy loans	808.3	823.8
Investment in unconsolidated entities	820.6	773.2
Other investments	2,765.6	2,882.7
Cash and cash equivalents	2,470.8	2,719.6	$ 2,564.8	$ 1,863.9
Accrued investment income	610.6	580.6
Premiums due and other receivables	1,469.8	1,361.9
Deferred acquisition costs	3,540.7	3,380.2	3,276.1	2,993.0
Property and equipment	759.5	699.0
Goodwill	1,068.8	1,020.8	1,009.0
Other intangibles	1,314.7	1,325.3
Separate account assets	159,272.7	139,832.6
Other assets	1,330.9	1,258.8
Total assets	253,941.2	228,014.3
Liabilities
Contractholder funds	38,082.5	37,953.6
Future policy benefits and claims	33,019.3	29,000.7
Other policyholder funds	922.3	890.4
Short-term debt	39.5	51.4
Long-term debt	3,178.4	3,125.7
Income taxes currently payable	16.7	12.9
Deferred income taxes	1,092.5	972.4
Separate account liabilities	159,272.7	139,832.6
Other liabilities	5,294.1	5,783.3
Total liabilities	240,918.0	217,623.0
Redeemable noncontrolling interest	101.3	97.5	85.7	58.0
Stockholders' equity
Common stock	4.7	4.7
Additional paid-in capital	9,925.2	9,686.0
Retained earnings (accumulated deficit)	9,482.9	7,720.4
Accumulated other comprehensive income (loss)	165.5	(675.2)
Treasury stock, at cost	(6,729.0)	(6,508.6)
Total stockholders' equity attributable to Principal Financial Group, Inc.	12,849.3	10,227.3
Noncontrolling interest	72.6	66.5
Total stockholders' equity	12,921.9	10,293.8	9,377.4	10,232.0
Total liabilities and stockholders' equity	253,941.2	228,014.3
Principal Financial Group, Inc. Parent Only
Assets
Fixed maturities, trading	351.8
Investment in unconsolidated entities	15,063.2	12,597.9
Other investments	10.2	9.8
Cash and cash equivalents	842.8	882.6	578.7	412.4
Accrued investment income	1.1
Other assets	389.7	573.7
Total assets	16,658.8	14,064.0
Liabilities
Long-term debt	3,128.1	3,126.4
Other liabilities	681.4	710.3
Total liabilities	3,809.5	3,836.7
Stockholders' equity
Common stock	4.7	4.7
Additional paid-in capital	9,925.2	9,686.0
Retained earnings (accumulated deficit)	9,482.9	7,720.4
Accumulated other comprehensive income (loss)	165.5	(675.2)
Treasury stock, at cost	(6,729.0)	(6,508.6)
Total stockholders' equity attributable to Principal Financial Group, Inc.	12,849.3	10,227.3
Total stockholders' equity	12,849.3	10,227.3
Total liabilities and stockholders' equity	16,658.8	14,064.0
Principal Financial Services, Inc. Only
Assets
Equity securities, trading	5.7
Investment in unconsolidated entities	15,449.7	12,532.4
Other investments	131.3	135.9
Cash and cash equivalents	617.4	1,203.4	730.5	907.2
Accrued investment income	0.2	0.1
Premiums due and other receivables	2.0	0.3
Other assets	94.8	185.6
Total assets	16,301.1	14,057.7
Liabilities
Long-term debt	412.7	142.1
Income taxes currently payable	37.3
Deferred income taxes	12.4
Other liabilities	894.1	1,411.3
Total liabilities	1,356.5	1,553.4
Stockholders' equity
Additional paid-in capital	9,053.5	9,010.9
Retained earnings (accumulated deficit)	5,311.7	3,724.3
Accumulated other comprehensive income (loss)	579.4	(230.9)
Total stockholders' equity attributable to Principal Financial Group, Inc.	14,944.6	12,504.3
Total stockholders' equity	14,944.6	12,504.3
Total liabilities and stockholders' equity	16,301.1	14,057.7
Principal Life Insurance Company and Other Subsidiaries Combined
Assets
Fixed maturities, available-for-sale	59,388.4	54,846.1
Fixed maturities, trading	214.2	398.4
Equity securities, available-for-sale	96.0	98.9
Equity securities, trading	1,764.9	1,413.4
Mortgage loans	14,150.5	13,230.2
Real estate	1,736.7	1,368.8
Policy loans	808.3	823.8
Investment in unconsolidated entities	706.7	697.5
Other investments	2,624.1	2,737.0
Cash and cash equivalents	2,001.1	2,114.8	2,413.3	1,598.0
Accrued investment income	609.3	580.5
Premiums due and other receivables	1,880.3	1,503.1
Deferred acquisition costs	3,540.7	3,380.2
Property and equipment	759.5	699.0
Goodwill	1,068.8	1,020.8
Other intangibles	1,314.7	1,325.3
Separate account assets	159,272.7	139,832.6
Other assets	1,619.4	1,200.9
Total assets	253,556.3	227,271.3
Liabilities
Contractholder funds	38,082.5	37,953.6
Future policy benefits and claims	33,019.3	29,000.7
Other policyholder funds	922.3	890.4
Short-term debt	39.5	127.9
Long-term debt	50.3	(0.8)
Income taxes currently payable	105.9	68.3
Deferred income taxes	1,705.1	1,619.3
Separate account liabilities	159,272.7	139,832.6
Other liabilities	4,760.7	4,962.1
Total liabilities	237,958.3	214,454.1
Redeemable noncontrolling interest	101.3	97.5
Stockholders' equity
Common stock	11.0	17.8
Additional paid-in capital	11,336.0	10,045.9
Retained earnings (accumulated deficit)	3,394.1	2,940.2
Accumulated other comprehensive income (loss)	685.0	(348.7)
Treasury stock, at cost	(2.0)	(2.0)
Total stockholders' equity attributable to Principal Financial Group, Inc.	15,424.1	12,653.2
Noncontrolling interest	72.6	66.5
Total stockholders' equity	15,496.7	12,719.7
Total liabilities and stockholders' equity	253,556.3	227,271.3
Principal Life Insurance Company and Other Subsidiaries Combined | Nonqualified benefit plans
Stockholders' equity
Assets held in Rabbi trusts	643.3	570.9
Liabilities held in Rabbi trusts	524.7	477.3
Eliminations, Shelf Registration Debt Guarantor
Assets
Investment in unconsolidated entities	(30,399.0)	(25,054.6)
Cash and cash equivalents	(990.5)	(1,481.2)	$ (1,157.7)	$ (1,053.7)
Premiums due and other receivables	(412.5)	(141.5)
Other assets	(773.0)	(701.4)
Total assets	(32,575.0)	(27,378.7)
Liabilities
Short-term debt		(76.5)
Long-term debt	(412.7)	(142.0)
Income taxes currently payable	(126.5)	(55.4)
Deferred income taxes	(625.0)	(646.9)
Other liabilities	(1,042.1)	(1,300.4)
Total liabilities	(2,206.3)	(2,221.2)
Stockholders' equity
Common stock	(11.0)	(17.8)
Additional paid-in capital	(20,389.5)	(19,056.8)
Retained earnings (accumulated deficit)	(8,705.8)	(6,664.5)
Accumulated other comprehensive income (loss)	(1,264.4)	579.6
Treasury stock, at cost	2.0	2.0
Total stockholders' equity attributable to Principal Financial Group, Inc.	(30,368.7)	(25,157.5)
Total stockholders' equity	(30,368.7)	(25,157.5)
Total liabilities and stockholders' equity	$ (32,575.0)	$ (27,378.7)