Condensed Consolidating Financial Information - Guarantor, Statements of Financial Position (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assets
Fixed maturities, available-for-sale	$ 60,108.5	$ 59,388.4
Fixed maturities, trading	636.1	566.0
Equity securities	1,843.7	1,866.6
Mortgage loans	15,336.9	14,150.5
Real estate	1,729.7	1,736.7
Policy loans	801.4	808.3
Investment in unconsolidated entities	868.8	820.6
Other investments	3,441.5	2,765.6
Cash and cash equivalents	2,977.5	2,470.8	$ 2,719.6	$ 2,564.8
Accrued investment income	636.2	610.6
Premiums due and other receivables	1,413.1	1,469.8
Deferred acquisition costs	3,693.5	3,540.7	3,380.2	3,276.1
Property and equipment	767.3	759.5
Goodwill	1,100.0	1,068.8	1,020.8
Other intangibles	1,315.1	1,314.7
Separate account assets	144,987.9	159,272.7
Other assets	1,378.9	1,330.9
Total assets	243,036.1	253,941.2
Liabilities
Contractholder funds	39,699.7	38,082.5
Future policy benefits and claims	35,664.8	33,019.3
Other policyholder funds	888.4	922.3
Short-term debt	42.9	39.5
Long-term debt	3,259.6	3,178.4
Income taxes currently payable	25.3	16.7
Deferred income taxes	958.4	1,092.5
Separate account liabilities	144,987.9	159,272.7
Other liabilities	5,661.9	5,294.1
Total liabilities	231,188.9	240,918.0
Redeemable noncontrolling interest	391.2	101.3	97.5	85.7
Stockholders' equity
Common stock	4.8	4.7
Additional paid-in capital	10,060.7	9,925.2
Retained earnings (accumulated deficit)	10,290.2	9,482.9
Accumulated other comprehensive income (loss)	(1,565.1)	165.5
Treasury stock, at cost	(7,400.6)	(6,729.0)
Total stockholders' equity attributable to Principal Financial Group, Inc.	11,390.0	12,849.3
Noncontrolling interest	66.0	72.6
Total stockholders' equity	11,456.0	12,921.9	10,293.8	9,377.4
Total liabilities and stockholders' equity	243,036.1	253,941.2
Principal Financial Group, Inc. Parent Only
Assets
Fixed maturities, available-for-sale	211.5
Fixed maturities, trading	308.9	351.8
Investment in unconsolidated entities	13,862.4	15,063.2
Other investments	10.7	10.2
Cash and cash equivalents	334.9	842.8	882.6	578.7
Accrued investment income	1.4	1.1
Other assets	375.0	389.7
Total assets	15,104.8	16,658.8
Liabilities
Long-term debt	3,129.8	3,128.1
Other liabilities	585.0	681.4
Total liabilities	3,714.8	3,809.5
Stockholders' equity
Common stock	4.8	4.7
Additional paid-in capital	10,060.7	9,925.2
Retained earnings (accumulated deficit)	10,290.2	9,482.9
Accumulated other comprehensive income (loss)	(1,565.1)	165.5
Treasury stock, at cost	(7,400.6)	(6,729.0)
Total stockholders' equity attributable to Principal Financial Group, Inc.	11,390.0	12,849.3
Total stockholders' equity	11,390.0	12,849.3
Total liabilities and stockholders' equity	15,104.8	16,658.8
Principal Life Insurance Company Only
Assets
Fixed maturities, available-for-sale	53,401.7	52,632.1
Fixed maturities, trading	165.5	49.1
Equity securities	84.8	100.1
Mortgage loans	14,478.0	13,389.6
Real estate	1.9	3.0
Policy loans	755.9	765.7
Investment in unconsolidated entities	2,324.4	2,658.2
Other investments	5,549.1	4,986.0
Cash and cash equivalents	1,323.9	426.7	675.1	1,127.9
Accrued investment income	563.2	538.8
Premiums due and other receivables	1,818.1	1,720.1
Deferred acquisition costs	3,680.2	3,331.5
Property and equipment	661.3	653.3
Goodwill	54.3	54.3
Other intangibles	19.7	21.6
Separate account assets	107,343.0	117,300.8
Other assets	1,135.1	1,177.5
Total assets	193,360.1	199,808.4
Liabilities
Contractholder funds	36,861.7	35,330.2
Future policy benefits and claims	30,690.3	27,794.0
Other policyholder funds	725.7	794.5
Deferred income taxes	405.3	731.6
Separate account liabilities	107,343.0	117,300.8
Other liabilities	8,503.2	7,910.9
Total liabilities	184,529.2	189,862.0
Stockholders' equity
Common stock	2.5	2.5
Additional paid-in capital	6,331.6	6,346.0
Retained earnings (accumulated deficit)	2,441.2	2,238.1
Accumulated other comprehensive income (loss)	55.6	1,359.8
Total stockholders' equity attributable to Principal Financial Group, Inc.	8,830.9	9,946.4
Total stockholders' equity	8,830.9	9,946.4
Total liabilities and stockholders' equity	193,360.1	199,808.4
Principal Financial Services, Inc. and Other Subsidiaries Combined
Assets
Fixed maturities, available-for-sale	6,857.9	7,120.3
Fixed maturities, trading	161.7	165.1
Equity securities	1,758.9	1,766.5
Mortgage loans	1,615.5	1,449.6
Real estate	1,727.8	1,733.7
Policy loans	45.5	42.6
Investment in unconsolidated entities	6,853.8	7,491.4
Other investments	2,517.6	2,231.9
Cash and cash equivalents	2,014.8	1,526.4	2,082.8	1,253.7
Accrued investment income	77.5	77.7
Premiums due and other receivables	3,781.3	3,362.4
Deferred acquisition costs	13.3	209.2
Property and equipment	106.0	106.2
Goodwill	1,045.7	1,014.5
Other intangibles	1,295.4	1,293.1
Separate account assets	37,644.9	41,971.9
Other assets	4,418.3	4,346.0
Total assets	71,935.9	75,908.5
Liabilities
Contractholder funds	3,189.2	3,092.8
Future policy benefits and claims	6,128.6	6,155.0
Other policyholder funds	165.3	128.6
Short-term debt	42.9	39.5
Long-term debt	851.2	710.4
Income taxes currently payable	80.7	166.2
Deferred income taxes	1,056.4	986.9
Separate account liabilities	37,644.9	41,971.9
Other liabilities	8,586.5	7,533.6
Total liabilities	57,745.7	60,784.9
Redeemable noncontrolling interest	391.2	101.3
Stockholders' equity
Additional paid-in capital	9,461.2	9,053.5
Retained earnings (accumulated deficit)	5,855.0	5,311.7
Accumulated other comprehensive income (loss)	(1,583.2)	579.4
Total stockholders' equity attributable to Principal Financial Group, Inc.	13,733.0	14,944.6
Noncontrolling interest	66.0	77.7
Total stockholders' equity	13,799.0	15,022.3
Total liabilities and stockholders' equity	71,935.9	75,908.5
Principal Financial Services, Inc. and Other Subsidiaries Combined | Nonqualified benefit plans
Stockholders' equity
Assets held in Rabbi trusts	671.7	643.3
Liabilities held in Rabbi trusts	542.4	524.7
Eliminations, Notes Guarantor
Assets
Fixed maturities, available-for-sale	(362.6)	(364.0)
Mortgage loans	(756.6)	(688.7)
Investment in unconsolidated entities	(22,171.8)	(24,392.2)
Other investments	(4,635.9)	(4,462.5)
Cash and cash equivalents	(696.1)	(325.1)	$ (920.9)	$ (395.5)
Accrued investment income	(5.9)	(7.0)
Premiums due and other receivables	(4,186.3)	(3,612.7)
Other assets	(4,549.5)	(4,582.3)
Total assets	(37,364.7)	(38,434.5)
Liabilities
Contractholder funds	(351.2)	(340.5)
Future policy benefits and claims	(1,154.1)	(929.7)
Other policyholder funds	(2.6)	(0.8)
Long-term debt	(721.4)	(660.1)
Income taxes currently payable	(55.4)	(149.5)
Deferred income taxes	(503.3)	(626.0)
Other liabilities	(12,012.8)	(10,831.8)
Total liabilities	(14,800.8)	(13,538.4)
Stockholders' equity
Common stock	(2.5)	(2.5)
Additional paid-in capital	(15,792.8)	(15,399.5)
Retained earnings (accumulated deficit)	(8,296.2)	(7,549.8)
Accumulated other comprehensive income (loss)	1,527.6	(1,939.2)
Total stockholders' equity attributable to Principal Financial Group, Inc.	(22,563.9)	(24,891.0)
Noncontrolling interest		(5.1)
Total stockholders' equity	(22,563.9)	(24,896.1)
Total liabilities and stockholders' equity	$ (37,364.7)	$ (38,434.5)