Condensed Consolidating Financial Information - Shelf Registration Guarantor, Statements of Financial Position (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015	Dec. 31, 2014
Assets
Fixed maturities, available-for-sale	$ 55,281.3	$ 49,966.5
Fixed maturities, trading	656.9	686.8
Equity securities, available-for-sale	105.2	104.5
Equity securities, trading	1,331.1	1,202.7
Mortgage loans	12,685.8	12,339.4
Real estate	1,474.0	1,451.8
Policy loans	831.6	817.1
Investment in unconsolidated entities	772.0	632.7
Other investments	3,549.5	2,619.0
Cash and cash equivalents	2,255.4	2,564.8	$ 2,264.4	$ 1,863.9
Accrued investment income	563.0	545.6
Premiums due and other receivables	1,504.8	1,429.3
Deferred acquisition costs	3,173.7	3,276.1
Property and equipment	667.9	633.8
Goodwill	1,037.2	1,009.0
Other intangibles	1,358.5	1,359.2
Separate account assets	134,736.0	136,978.9
Other assets	1,090.3	1,043.1
Total assets	223,074.2	218,660.3
Liabilities
Contractholder funds	37,781.0	35,716.1
Future policy benefits and claims	27,996.1	25,856.5
Other policyholder funds	888.4	805.4
Short-term debt	25.9	181.1
Long-term debt	3,270.2	3,265.2
Income taxes currently payable	10.2	18.4
Deferred income taxes	1,234.7	697.2
Separate account liabilities	134,736.0	136,978.9
Other liabilities	6,226.9	5,678.4
Total liabilities	212,169.4	209,197.2
Redeemable noncontrolling interest	337.1	85.7	71.6	58.0
Stockholders' equity
Common stock	4.7	4.7
Additional paid-in capital	9,615.7	9,544.8
Retained earnings (accumulated deficit)	7,339.6	6,875.9
Accumulated other comprehensive income (loss)	(19.3)	(882.5)
Treasury stock, at cost	(6,441.0)	(6,231.3)
Total stockholders' equity attributable to Principal Financial Group, Inc.	10,499.7	9,311.6
Noncontrolling interest	68.0	65.8
Total stockholders' equity	10,567.7	9,377.4	9,762.6	10,232.0
Total liabilities and stockholders' equity	223,074.2	218,660.3
Principal Financial Group, Inc. Parent Only
Assets
Investment in unconsolidated entities	13,275.3	12,223.4
Other investments	9.7	9.7
Cash and cash equivalents	692.3	578.7	762.9	412.4
Other assets	444.3	458.0
Total assets	14,421.6	13,269.8
Liabilities
Long-term debt	3,224.8	3,223.8
Other liabilities	697.1	734.4
Total liabilities	3,921.9	3,958.2
Stockholders' equity
Common stock	4.7	4.7
Additional paid-in capital	9,615.7	9,544.8
Retained earnings (accumulated deficit)	7,339.6	6,875.9
Accumulated other comprehensive income (loss)	(19.3)	(882.5)
Treasury stock, at cost	(6,441.0)	(6,231.3)
Total stockholders' equity attributable to Principal Financial Group, Inc.	10,499.7	9,311.6
Total stockholders' equity	10,499.7	9,311.6
Total liabilities and stockholders' equity	14,421.6	13,269.8
Principal Financial Services, Inc. Only
Assets
Investment in unconsolidated entities	13,308.2	12,209.1
Other investments	103.6	185.9
Cash and cash equivalents	807.9	730.5	804.3	907.2
Accrued investment income	2.3
Premiums due and other receivables	90.6	0.1
Other assets	193.1	205.2
Total assets	14,505.7	13,330.8
Liabilities
Long-term debt	150.0	156.0
Income taxes currently payable	2.4	3.2
Other liabilities	1,166.0	1,032.9
Total liabilities	1,318.4	1,192.1
Stockholders' equity
Additional paid-in capital	9,044.3	9,000.0
Retained earnings (accumulated deficit)	3,697.8	3,522.3
Accumulated other comprehensive income (loss)	445.2	(383.6)
Total stockholders' equity attributable to Principal Financial Group, Inc.	13,187.3	12,138.7
Total stockholders' equity	13,187.3	12,138.7
Total liabilities and stockholders' equity	14,505.7	13,330.8
Principal Life Insurance Company and Other Subsidiaries Combined
Assets
Fixed maturities, available-for-sale	55,281.3	49,966.5
Fixed maturities, trading	656.9	686.8
Equity securities, available-for-sale	105.2	104.5
Equity securities, trading	1,331.1	1,202.7
Mortgage loans	12,685.8	12,339.4
Real estate	1,474.0	1,451.8
Policy loans	831.6	817.1
Investment in unconsolidated entities	710.7	583.2
Other investments	3,436.2	2,423.4
Cash and cash equivalents	1,989.1	2,413.3	1,847.0	1,598.0
Accrued investment income	560.7	545.6
Premiums due and other receivables	1,630.9	1,584.6
Deferred acquisition costs	3,173.7	3,276.1
Property and equipment	667.9	633.8
Goodwill	1,037.2	1,009.0
Other intangibles	1,358.5	1,359.2
Separate account assets	134,736.0	136,978.9
Other assets	1,096.2	1,065.4
Total assets	222,763.0	218,441.3
Liabilities
Contractholder funds	37,781.0	35,716.1
Future policy benefits and claims	27,996.1	25,856.5
Other policyholder funds	888.4	805.4
Short-term debt	109.5	290.0
Long-term debt	112.4	41.4
Income taxes currently payable	74.3	69.6
Deferred income taxes	1,813.3	1,325.2
Separate account liabilities	134,736.0	136,978.9
Other liabilities	5,162.1	4,912.9
Total liabilities	208,673.1	205,996.0
Redeemable noncontrolling interest	337.1	85.7
Stockholders' equity
Common stock	17.8	17.8
Additional paid-in capital	10,311.3	9,888.7
Retained earnings (accumulated deficit)	3,024.5	2,905.9
Accumulated other comprehensive income (loss)	333.2	(516.6)
Treasury stock, at cost	(2.0)	(2.0)
Total stockholders' equity attributable to Principal Financial Group, Inc.	13,684.8	12,293.8
Noncontrolling interest	68.0	65.8
Total stockholders' equity	13,752.8	12,359.6
Total liabilities and stockholders' equity	222,763.0	218,441.3
Eliminations, Shelf Registration Debt Guarantor
Assets
Investment in unconsolidated entities	(26,522.2)	(24,383.0)
Cash and cash equivalents	(1,233.9)	(1,157.7)	$ (1,149.8)	$ (1,053.7)
Premiums due and other receivables	(216.7)	(155.4)
Other assets	(643.3)	(685.5)
Total assets	(28,616.1)	(26,381.6)
Liabilities
Short-term debt	(83.6)	(108.9)
Long-term debt	(217.0)	(156.0)
Income taxes currently payable	(66.5)	(54.4)
Deferred income taxes	(578.6)	(628.0)
Other liabilities	(798.3)	(1,001.8)
Total liabilities	(1,744.0)	(1,949.1)
Stockholders' equity
Common stock	(17.8)	(17.8)
Additional paid-in capital	(19,355.6)	(18,888.7)
Retained earnings (accumulated deficit)	(6,722.3)	(6,428.2)
Accumulated other comprehensive income (loss)	(778.4)	900.2
Treasury stock, at cost	2.0	2.0
Total stockholders' equity attributable to Principal Financial Group, Inc.	(26,872.1)	(24,432.5)
Total stockholders' equity	(26,872.1)	(24,432.5)
Total liabilities and stockholders' equity	$ (28,616.1)	$ (26,381.6)