Schedule II - Condensed Financial Information of Registrant (Parent Only) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Assets
Cash and cash equivalents	$ 1,863.9	$ 2,371.8	$ 4,177.2	$ 2,833.9
Other investments	3,209.8	2,944.4
Deferred income taxes	90.8	49.6
Investment in subsidiary	815.8	880.0
Total assets	219,087.0	208,191.4
Liabilities
Long-term debt	2,531.2	2,601.4
Total liabilities	208,797.0	198,167.2
Stockholders' equity
Common stock, par value $.01 per share - 2,500.0 million shares authorized, 462.7 million and 459.3 million shares issued, and 293.9 million and 295.2 million shares outstanding in 2014 and 2013	4.6	4.6
Additional paid-in capital	9,945.5	9,798.9
Retained earnings (accumulated deficit)	6,114.1	5,405.4
Accumulated other comprehensive income (loss)	50.4	183.2
Treasury stock, at cost (168.8 million and 164.1 million shares in 2014 and 2013)	(5,930.7)	(5,708.0)
Total stockholders' equity attributable to Principal Financial Group, Inc.	10,184.0	9,684.2
Total liabilities and stockholders' equity	219,087.0	208,191.4
Series A
Stockholders' equity
Preferred stock, value	0	0
Series B
Stockholders' equity
Preferred stock, value	0.1	0.1
Principal Financial Group, Inc. Parent Only
Assets
Cash and cash equivalents	412.4	131.5	207.1	226.7
Other investments	9.5	9.3
Income taxes receivable	21.8	22.2
Deferred income taxes	405.5	17.2
Amounts receivable from subsidiary	5.4	0.8
Other assets	18.0	19.0
Investment in subsidiary	12,446.5	11,956.2
Total assets	13,319.1	12,156.2
Liabilities
Amounts payable to subsidiary		1.4
Long-term debt	2,448.9	2,448.8
Accrued investment payable	21.8	21.8
Pension liability	664.4
Total liabilities	3,135.1	2,472.0
Stockholders' equity
Common stock, par value $.01 per share - 2,500.0 million shares authorized, 462.7 million and 459.3 million shares issued, and 293.9 million and 295.2 million shares outstanding in 2014 and 2013	4.6	4.6
Additional paid-in capital	9,945.5	9,798.9
Retained earnings (accumulated deficit)	6,114.1	5,405.4
Accumulated other comprehensive income (loss)	50.4	183.2
Treasury stock, at cost (168.8 million and 164.1 million shares in 2014 and 2013)	(5,930.7)	(5,708.0)
Total stockholders' equity attributable to Principal Financial Group, Inc.	10,184.0	9,684.2
Total liabilities and stockholders' equity	13,319.1	12,156.2
Principal Financial Group, Inc. Parent Only | Series B
Stockholders' equity
Preferred stock, value	$ 0.1	$ 0.1