Condensed Consolidating Financial Information - Shelf Registration Guarantor, Statements of Financial Position (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Assets
Fixed maturities, available-for-sale	$ 49,966.5	$ 49,670.8
Fixed maturities, trading	686.8	604.6
Equity securities, available-for-sale	104.5	123.0
Equity securities, trading	1,202.7	840.2
Mortgage loans	12,339.4	11,811.6
Real estate	1,451.8	1,344.6
Policy loans	817.1	829.2
Investment in unconsolidated entities	632.7	815.8
Other investments	2,619.0	2,394.0
Cash and cash equivalents	2,564.8	1,863.9	$ 2,371.8	$ 4,177.2
Accrued investment income	545.6	505.9
Premiums due and other receivables	1,429.3	1,213.0
Deferred acquisition costs	3,276.1	2,993.0	3,077.0	2,590.0
Property and equipment	633.8	590.2
Goodwill	1,009.0	1,007.4	1,100.3
Other intangibles	1,359.2	1,323.5
Separate account assets	136,978.9	140,072.8
Other assets	1,068.7	1,083.5
Total assets	218,685.9	219,087.0
Liabilities
Contractholder funds	35,716.1	34,726.7
Future policy benefits and claims	25,856.5	24,036.6
Other policyholder funds	805.4	812.7
Short-term debt	181.1	28.0
Long-term debt	3,290.8	2,531.2
Income taxes currently payable	18.4	11.5
Deferred income taxes	697.2	1,035.3
Separate account liabilities	136,978.9	140,072.8
Other liabilities	5,678.4	5,542.2
Total liabilities	209,222.8	208,797.0
Redeemable noncontrolling interest	85.7	58.0	247.2	60.4
Stockholders' equity
Common stock	4.7	4.6
Additional paid-in capital	9,544.8	9,945.5
Retained earnings (accumulated deficit)	6,875.9	6,114.1
Accumulated other comprehensive income	(882.5)	50.4
Treasury stock, at cost	(6,231.3)	(5,930.7)
Total stockholders' equity attributable to Principal Financial Group, Inc.	9,311.6	10,184.0
Noncontrolling interest	65.8	48.0
Total stockholders' equity	9,377.4	10,232.0	9,777.0	9,703.4
Total liabilities and stockholders' equity	218,685.9	219,087.0
Principal Financial Group, Inc. Parent Only
Assets
Investment in unconsolidated entities	12,223.4	12,446.5
Other investments	9.7	9.5
Cash and cash equivalents	578.7	412.4	131.5	207.1
Other assets	482.1	450.7
Total assets	13,293.9	13,319.1
Liabilities
Long-term debt	3,247.9	2,448.9
Other liabilities	734.4	686.2
Total liabilities	3,982.3	3,135.1
Stockholders' equity
Common stock	4.7	4.6
Additional paid-in capital	9,544.8	9,945.5
Retained earnings (accumulated deficit)	6,875.9	6,114.1
Accumulated other comprehensive income	(882.5)	50.4
Treasury stock, at cost	(6,231.3)	(5,930.7)
Total stockholders' equity attributable to Principal Financial Group, Inc.	9,311.6	10,184.0
Total stockholders' equity	9,311.6	10,184.0
Total liabilities and stockholders' equity	13,293.9	13,319.1
Principal Financial Services, Inc. Only
Assets
Investment in unconsolidated entities	12,209.1	12,042.5
Other investments	185.9	128.2
Cash and cash equivalents	730.5	907.2	688.7	612.5
Premiums due and other receivables	0.1	0.3
Other assets	205.2	167.6
Total assets	13,330.8	13,245.8
Liabilities
Long-term debt	156.0
Income taxes currently payable	3.2	4.3
Other liabilities	1,032.9	872.8
Total liabilities	1,192.1	877.1
Stockholders' equity
Additional paid-in capital	9,000.0	8,964.2
Retained earnings (accumulated deficit)	3,522.3	2,858.6
Accumulated other comprehensive income	(383.6)	545.9
Total stockholders' equity attributable to Principal Financial Group, Inc.	12,138.7	12,368.7
Total stockholders' equity	12,138.7	12,368.7
Total liabilities and stockholders' equity	13,330.8	13,245.8
Principal Life Insurance Company and Other Subsidiaries Combined
Assets
Fixed maturities, available-for-sale	49,966.5	49,670.8
Fixed maturities, trading	686.8	604.6
Equity securities, available-for-sale	104.5	123.0
Equity securities, trading	1,202.7	840.2
Mortgage loans	12,339.4	11,811.6
Real estate	1,451.8	1,344.6
Policy loans	817.1	829.2
Investment in unconsolidated entities	583.2	781.9
Other investments	2,423.4	2,256.3
Cash and cash equivalents	2,413.3	1,598.0	2,384.0	4,241.3
Accrued investment income	545.6	505.9
Premiums due and other receivables	1,584.6	1,211.8
Deferred acquisition costs	3,276.1	2,993.0
Property and equipment	633.8	590.2
Goodwill	1,009.0	1,007.4
Other intangibles	1,359.2	1,323.5
Separate account assets	136,978.9	140,072.8
Other assets	1,066.9	1,117.7
Total assets	218,442.8	218,682.5
Liabilities
Contractholder funds	35,716.1	34,726.7
Future policy benefits and claims	25,856.5	24,036.6
Other policyholder funds	805.4	812.7
Short-term debt	290.0	182.5
Long-term debt	42.9	82.3
Income taxes currently payable	69.6	54.1
Deferred income taxes	1,325.2	1,626.1
Separate account liabilities	136,978.9	140,072.8
Other liabilities	4,912.9	4,862.4
Total liabilities	205,997.5	206,456.2
Redeemable noncontrolling interest	85.7	58.0
Stockholders' equity
Common stock	17.8	17.8
Additional paid-in capital	9,888.7	9,341.0
Retained earnings (accumulated deficit)	2,905.9	2,283.2
Accumulated other comprehensive income	(516.6)	480.3
Treasury stock, at cost	(2.0)	(2.0)
Total stockholders' equity attributable to Principal Financial Group, Inc.	12,293.8	12,120.3
Noncontrolling interest	65.8	48.0
Total stockholders' equity	12,359.6	12,168.3
Total liabilities and stockholders' equity	218,442.8	218,682.5
Eliminations, Shelf Registration Debt Guarantor
Assets
Investment in unconsolidated entities	(24,383.0)	(24,455.1)
Cash and cash equivalents	(1,157.7)	(1,053.7)	$ (832.4)	$ (883.7)
Premiums due and other receivables	(155.4)	0.9
Other assets	(685.5)	(652.5)
Total assets	(26,381.6)	(26,160.4)
Liabilities
Short-term debt	(108.9)	(154.5)
Long-term debt	(156.0)
Income taxes currently payable	(54.4)	(46.9)
Deferred income taxes	(628.0)	(590.8)
Other liabilities	(1,001.8)	(879.2)
Total liabilities	(1,949.1)	(1,671.4)
Stockholders' equity
Common stock	(17.8)	(17.8)
Additional paid-in capital	(18,888.7)	(18,305.2)
Retained earnings (accumulated deficit)	(6,428.2)	(5,141.8)
Accumulated other comprehensive income	900.2	(1,026.2)
Treasury stock, at cost	2.0	2.0
Total stockholders' equity attributable to Principal Financial Group, Inc.	(24,432.5)	(24,489.0)
Total stockholders' equity	(24,432.5)	(24,489.0)
Total liabilities and stockholders' equity	$ (26,381.6)	$ (26,160.4)
Series A
Stockholders' equity
Preferred stock, value
Series B
Stockholders' equity
Preferred stock, value		$ 0.1
Series B | Principal Financial Group, Inc. Parent Only
Stockholders' equity
Preferred stock, value		$ 0.1