Condensed Consolidating Financial Information - 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 Life Insurance Company Only
Assets
Fixed maturities, available-for-sale	52,632.1	48,672.1
Fixed maturities, trading	49.1	135.6
Equity securities, available-for-sale	94.1	96.3
Equity securities, trading	6.0	8.2
Mortgage loans	13,389.6	12,460.7
Real estate	3.0	4.4
Policy loans	765.7	784.8
Investment in unconsolidated entities	2,658.2	2,071.1
Other investments	4,986.0	4,740.0
Cash and cash equivalents	426.7	675.1	1,127.9	602.7
Accrued investment income	538.8	513.7
Premiums due and other receivables	1,720.1	1,538.0
Deferred acquisition costs	3,331.5	3,184.2
Property and equipment	653.3	610.4
Goodwill	54.3	54.3
Other intangibles	21.6	23.5
Separate account assets	117,300.8	103,661.9
Other assets	1,177.5	969.5
Total assets	199,808.4	180,203.8
Liabilities
Contractholder funds	35,330.2	35,337.7
Future policy benefits and claims	27,794.0	24,392.6
Other policyholder funds	794.5	780.7
Deferred income taxes	731.6	533.6
Separate account liabilities	117,300.8	103,661.9
Other liabilities	7,910.9	7,300.9
Total liabilities	189,862.0	172,007.4
Stockholders' equity
Common stock	2.5	2.5
Additional paid-in capital	6,346.0	5,305.6
Retained earnings (accumulated deficit)	2,238.1	2,139.9
Accumulated other comprehensive income (loss)	1,359.8	748.4
Total stockholders' equity attributable to Principal Financial Group, Inc.	9,946.4	8,196.4
Total stockholders' equity	9,946.4	8,196.4
Total liabilities and stockholders' equity	199,808.4	180,203.8
Principal Financial Services, Inc. and Other Subsidiaries Combined
Assets
Fixed maturities, available-for-sale	7,120.3	6,559.9
Fixed maturities, trading	165.1	262.8
Equity securities, available-for-sale	1.9	2.6
Equity securities, trading	1,764.6	1,405.2
Mortgage loans	1,449.6	1,289.4
Real estate	1,733.7	1,364.4
Policy loans	42.6	39.0
Investment in unconsolidated entities	7,491.4	6,493.7
Other investments	2,231.9	1,783.0
Cash and cash equivalents	1,526.4	2,082.8	1,253.7	1,253.6
Accrued investment income	77.7	74.5
Premiums due and other receivables	3,362.4	2,836.0
Deferred acquisition costs	209.2	196.0
Property and equipment	106.2	88.6
Goodwill	1,014.5	966.5
Other intangibles	1,293.1	1,301.8
Separate account assets	41,971.9	36,170.7
Other assets	4,346.0	3,507.7
Total assets	75,908.5	66,424.6
Liabilities
Contractholder funds	3,092.8	2,949.2
Future policy benefits and claims	6,155.0	5,312.1
Other policyholder funds	128.6	111.0
Short-term debt	39.5	51.4
Long-term debt	710.4	495.1
Income taxes currently payable	166.2	124.3
Deferred income taxes	986.9	1,111.9
Separate account liabilities	41,971.9	36,170.7
Other liabilities	7,533.6	7,425.9
Total liabilities	60,784.9	53,751.6
Redeemable noncontrolling interest	101.3	97.5
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
Noncontrolling interest	77.7	71.2
Total stockholders' equity	15,022.3	12,575.5
Total liabilities and stockholders' equity	75,908.5	66,424.6
Principal Financial Services, Inc. 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, Notes Guarantor
Assets
Fixed maturities, available-for-sale	(364.0)	(385.9)
Mortgage loans	(688.7)	(519.9)
Investment in unconsolidated entities	(24,392.2)	(20,389.5)
Other investments	(4,462.5)	(3,650.1)
Cash and cash equivalents	(325.1)	(920.9)	$ (395.5)	$ (404.8)
Accrued investment income	(7.0)	(7.6)
Premiums due and other receivables	(3,612.7)	(3,012.1)
Other assets	(4,582.3)	(3,792.1)
Total assets	(38,434.5)	(32,678.1)
Liabilities
Contractholder funds	(340.5)	(333.3)
Future policy benefits and claims	(929.7)	(704.0)
Other policyholder funds	(0.8)	(1.3)
Long-term debt	(660.1)	(495.8)
Income taxes currently payable	(149.5)	(111.4)
Deferred income taxes	(626.0)	(673.1)
Other liabilities	(10,831.8)	(9,653.8)
Total liabilities	(13,538.4)	(11,972.7)
Stockholders' equity
Common stock	(2.5)	(2.5)
Additional paid-in capital	(15,399.5)	(14,316.5)
Retained earnings (accumulated deficit)	(7,549.8)	(5,864.2)
Accumulated other comprehensive income (loss)	(1,939.2)	(517.5)
Total stockholders' equity attributable to Principal Financial Group, Inc.	(24,891.0)	(20,700.7)
Noncontrolling interest	(5.1)	(4.7)
Total stockholders' equity	(24,896.1)	(20,705.4)
Total liabilities and stockholders' equity	$ (38,434.5)	$ (32,678.1)