Condensed Consolidating Financial Information (Details 4) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014	Dec. 31, 2013
Assets
Fixed maturities, available-for-sale	$ 48,949.7	$ 49,670.8
Fixed maturities, trading	560.3	604.6
Equity securities, available-for-sale	176.1	123.0
Equity securities, trading	891.8	840.2
Mortgage loans	12,070.7	11,811.6
Real estate	1,435.1	1,344.6
Policy loans	824.9	829.2
Investment in unconsolidated entities	695.6	815.8
Other investments	2,572.4	2,394.0
Cash and cash equivalents	2,264.4	1,863.9	$ 1,742.3	$ 2,371.8
Accrued investment income	498.7	505.9
Premiums due and other receivables	1,282.7	1,213.0
Deferred acquisition costs	3,162.4	2,993.0
Property and equipment	601.9	590.2
Goodwill	970.7	1,007.4
Other intangibles	1,268.8	1,323.5
Separate account assets	142,993.6	140,072.8
Other assets	1,050.9	1,083.5
Total assets	222,270.7	219,087.0
Liabilities
Contractholder funds	34,041.8	34,726.7
Future policy benefits and claims	24,952.7	24,036.6
Other policyholder funds	807.0	812.7
Short-term debt	26.6	28.0
Long-term debt	3,315.9	2,531.2
Income taxes currently payable	16.0	11.5
Deferred income taxes	819.7	1,035.3
Separate account liabilities	142,993.6	140,072.8
Other liabilities	5,463.2	5,542.2
Total liabilities	212,436.5	208,797.0
Redeemable noncontrolling interest	71.6	58.0	282.4	247.2
Stockholders' equity
Common stock	4.6	4.6
Additional paid-in capital	9,473.8	9,945.5
Retained earnings (accumulated deficit)	6,547.9	6,114.1
Accumulated other comprehensive income	(326.6)	50.4
Treasury stock, at cost	(6,006.3)	(5,930.7)
Total stockholders' equity attributable to Principal Financial Group, Inc.	9,693.4	10,184.0
Noncontrolling interest	69.2	48.0
Total stockholders' equity	9,762.6	10,232.0	10,459.3	9,777.0
Total liabilities and stockholders' equity	222,270.7	219,087.0
Principal Financial Group, Inc. Parent Only
Assets
Investment in unconsolidated entities	12,374.3	12,446.5
Other investments	9.5	9.5
Cash and cash equivalents	762.9	412.4	90.7	131.5
Other assets	455.3	450.7
Total assets	13,602.0	13,319.1
Liabilities
Long-term debt	3,247.8	2,448.9
Other liabilities	660.8	686.2
Total liabilities	3,908.6	3,135.1
Stockholders' equity
Common stock	4.6	4.6
Additional paid-in capital	9,473.8	9,945.5
Retained earnings (accumulated deficit)	6,547.9	6,114.1
Accumulated other comprehensive income	(326.6)	50.4
Treasury stock, at cost	(6,006.3)	(5,930.7)
Total stockholders' equity attributable to Principal Financial Group, Inc.	9,693.4	10,184.0
Total stockholders' equity	9,693.4	10,184.0
Total liabilities and stockholders' equity	13,602.0	13,319.1
Principal Financial Services, Inc. Only
Assets
Equity securities, trading	4.7
Investment in unconsolidated entities	12,122.9	12,042.5
Other investments	131.6	128.2
Cash and cash equivalents	804.3	907.2	684.8	688.7
Accrued investment income	1.2
Premiums due and other receivables	0.9	0.3
Other assets	201.9	167.6
Total assets	13,267.5	13,245.8
Liabilities
Income taxes currently payable		4.3
Other liabilities	974.5	872.8
Total liabilities	974.5	877.1
Stockholders' equity
Additional paid-in capital	8,976.9	8,964.2
Retained earnings (accumulated deficit)	3,179.8	2,858.6
Accumulated other comprehensive income	136.3	545.9
Total stockholders' equity attributable to Principal Financial Group, Inc.	12,293.0	12,368.7
Total stockholders' equity	12,293.0	12,368.7
Total liabilities and stockholders' equity	13,267.5	13,245.8
Principal Life Insurance Company and Other Subsidiaries Combined
Assets
Fixed maturities, available-for-sale	48,949.7	49,670.8
Fixed maturities, trading	560.3	604.6
Equity securities, available-for-sale	176.1	123.0
Equity securities, trading	887.1	840.2
Mortgage loans	12,070.7	11,811.6
Real estate	1,435.1	1,344.6
Policy loans	824.9	829.2
Investment in unconsolidated entities	655.9	781.9
Other investments	2,431.3	2,256.3
Cash and cash equivalents	1,847.0	1,598.0	1,643.2	2,384.0
Accrued investment income	497.5	505.9
Premiums due and other receivables	1,281.0	1,211.8
Deferred acquisition costs	3,162.4	2,993.0
Property and equipment	601.9	590.2
Goodwill	970.7	1,007.4
Other intangibles	1,268.8	1,323.5
Separate account assets	142,993.6	140,072.8
Other assets	1,050.0	1,117.7
Total assets	221,664.0	218,682.5
Liabilities
Contractholder funds	34,041.8	34,726.7
Future policy benefits and claims	24,952.7	24,036.6
Other policyholder funds	807.0	812.7
Short-term debt	181.5	182.5
Long-term debt	68.1	82.3
Income taxes currently payable	68.9	54.1
Deferred income taxes	1,412.5	1,626.1
Separate account liabilities	142,993.6	140,072.8
Other liabilities	4,792.9	4,862.4
Total liabilities	209,319.0	206,456.2
Redeemable noncontrolling interest	71.6	58.0
Stockholders' equity
Common stock	17.8	17.8
Additional paid-in capital	9,432.7	9,341.0
Retained earnings (accumulated deficit)	2,713.8	2,283.2
Accumulated other comprehensive income	41.9	480.3
Treasury stock, at cost	(2.0)	(2.0)
Total stockholders' equity attributable to Principal Financial Group, Inc.	12,204.2	12,120.3
Noncontrolling interest	69.2	48.0
Total stockholders' equity	12,273.4	12,168.3
Total liabilities and stockholders' equity	221,664.0	218,682.5
Eliminations, Shelf Registration Debt Guarantor
Assets
Investment in unconsolidated entities	(24,457.5)	(24,455.1)
Cash and cash equivalents	(1,149.8)	(1,053.7)	$ (676.4)	$ (832.4)
Premiums due and other receivables	0.8	0.9
Other assets	(656.3)	(652.5)
Total assets	(26,262.8)	(26,160.4)
Liabilities
Short-term debt	(154.9)	(154.5)
Income taxes currently payable	(52.9)	(46.9)
Deferred income taxes	(592.8)	(590.8)
Other liabilities	(965.0)	(879.2)
Total liabilities	(1,765.6)	(1,671.4)
Stockholders' equity
Common stock	(17.8)	(17.8)
Additional paid-in capital	(18,409.6)	(18,305.2)
Retained earnings (accumulated deficit)	(5,893.6)	(5,141.8)
Accumulated other comprehensive income	(178.2)	(1,026.2)
Treasury stock, at cost	2.0	2.0
Total stockholders' equity attributable to Principal Financial Group, Inc.	(24,497.2)	(24,489.0)
Total stockholders' equity	(24,497.2)	(24,489.0)
Total liabilities and stockholders' equity	$ (26,262.8)	$ (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