Schedule II - Condensed Financial Information of Registrant (Parent Only) - Statements of Financial Position (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Assets
Fixed maturities, trading	$ 566.0	$ 398.4
Cash and cash equivalents	2,470.8	2,719.6	$ 2,564.8	$ 1,863.9
Other investments	3,586.2	3,655.9
Deferred income tax assets	48.4	103.9
Investment in unconsolidated entities	820.6	773.2
Total assets	253,941.2	228,014.3
Liabilities
Long-term debt	3,178.4	3,125.7
Total liabilities	240,918.0	217,623.0
Stockholders' equity
Common stock, par value $.01 per share - 2,500.0 million shares authorized, 474.1 million and 469.2 million shares issued, and 289.0 million and 287.7 million shares outstanding in 2017 and 2016	4.7	4.7
Additional paid-in capital	9,925.2	9,686.0
Retained earnings (accumulated deficit)	9,482.9	7,720.4
Accumulated other comprehensive income (loss)	165.5	(675.2)
Treasury stock, at cost (185.1 million and 181.5 million shares in 2017 and 2016)	(6,729.0)	(6,508.6)
Total stockholders' equity attributable to Principal Financial Group, Inc.	12,849.3	10,227.3
Total liabilities and stockholders' equity	253,941.2	228,014.3
Principal Financial Group, Inc. Parent Only
Assets
Fixed maturities, trading	351.8
Cash and cash equivalents	842.8	882.6	$ 578.7	$ 412.4
Other investments	10.2	9.8
Income taxes receivable	44.2	85.6
Deferred income tax assets	329.4	473.2
Amounts receivable from subsidiaries	4.4	4.5
Other assets	12.8	10.4
Investment in unconsolidated entities	15,063.2	12,597.9
Total assets	16,658.8	14,064.0
Liabilities
Long-term debt	3,128.1	3,126.4
Accrued investment payable	23.1	23.4
Pension liability	658.3	686.4
Other liabilities		0.5
Total liabilities	3,809.5	3,836.7
Stockholders' equity
Common stock, par value $.01 per share - 2,500.0 million shares authorized, 474.1 million and 469.2 million shares issued, and 289.0 million and 287.7 million shares outstanding in 2017 and 2016	4.7	4.7
Additional paid-in capital	9,925.2	9,686.0
Retained earnings (accumulated deficit)	9,482.9	7,720.4
Accumulated other comprehensive income (loss)	165.5	(675.2)
Treasury stock, at cost (185.1 million and 181.5 million shares in 2017 and 2016)	(6,729.0)	(6,508.6)
Total stockholders' equity attributable to Principal Financial Group, Inc.	12,849.3	10,227.3
Total liabilities and stockholders' equity	$ 16,658.8	$ 14,064.0