Schedule II - Condensed Financial Information of Registrant (Parent Only) - Statements of Financial Position (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Assets
Cash and cash equivalents	$ 2,564.8	$ 1,863.9	$ 2,371.8	$ 4,177.2
Other investments	3,251.7	3,209.8
Deferred income tax assets	96.0	90.8
Investment in subsidiary	632.7	815.8
Total assets	218,685.9	219,087.0
Liabilities
Long-term debt	3,290.8	2,531.2
Total liabilities	209,222.8	208,797.0
Stockholders' equity
Common stock, par value $.01 per share - 2,500.0 million shares authorized, 466.2 million and 462.7 million shares issued, and 291.4 million and 293.9 million shares outstanding in 2015 and 2014	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 (loss)	(882.5)	50.4
Treasury stock, at cost (174.8 million and 168.8 million shares in 2015 and 2014)	(6,231.3)	(5,930.7)
Total stockholders' equity attributable to Principal Financial Group, Inc.	9,311.6	10,184.0
Total liabilities and stockholders' equity	218,685.9	$ 219,087.0
Series A
Stockholders' equity
Preferred stock, value
Series B
Stockholders' equity
Preferred stock, value		$ 0.1
Principal Financial Group, Inc. Parent Only
Assets
Cash and cash equivalents	578.7	412.4	$ 131.5	$ 207.1
Other investments	9.7	9.5
Income taxes receivable	21.3	21.8
Deferred income tax assets	427.4	405.5
Amounts receivable from subsidiary	8.9	5.4
Other assets	24.5	18.0
Investment in subsidiary	12,223.4	12,446.5
Total assets	13,293.9	13,319.1
Liabilities
Long-term debt	3,247.9	2,448.9
Accrued investment payable	25.8	21.8
Pension liability	708.6	664.4
Total liabilities	3,982.3	3,135.1
Stockholders' equity
Common stock, par value $.01 per share - 2,500.0 million shares authorized, 466.2 million and 462.7 million shares issued, and 291.4 million and 293.9 million shares outstanding in 2015 and 2014	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 (loss)	(882.5)	50.4
Treasury stock, at cost (174.8 million and 168.8 million shares in 2015 and 2014)	(6,231.3)	(5,930.7)
Total stockholders' equity attributable to Principal Financial Group, Inc.	9,311.6	10,184.0
Total liabilities and stockholders' equity	$ 13,293.9	$ 13,319.1
Principal Financial Group, Inc. Parent Only | Series A
Stockholders' equity
Preferred stock, value
Principal Financial Group, Inc. Parent Only | Series B
Stockholders' equity
Preferred stock, value		$ 0.1