Schedule II - Condensed Financial Information of Registrant (Parent Only) - Statements of Financial Position (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assets
Fixed maturities, available-for-sale	$ 60,108.5	$ 59,388.4
Fixed maturities, trading	636.1	566.0
Cash and cash equivalents	2,977.5	2,470.8	$ 2,719.6	$ 2,564.8
Other investments	4,310.3	3,586.2
Deferred income tax assets	141.3	48.4
Investment in unconsolidated entities	868.8	820.6
Total assets	243,036.1	253,941.2
Liabilities
Long-term debt	3,259.6	3,178.4
Total liabilities	231,188.9	240,918.0
Stockholders' equity
Common stock, par value $.01 per share - 2,500.0 million shares authorized, 476.7 million and 474.1 million shares issued, and 279.5 million and 289.0 million shares outstanding in 2018 and 2017	4.8	4.7
Additional paid-in capital	10,060.7	9,925.2
Retained earnings (accumulated deficit)	10,290.2	9,482.9
Accumulated other comprehensive income (loss)	(1,565.1)	165.5
Treasury stock, at cost (197.2 million and 185.1 million shares in 2018 and 2017)	(7,400.6)	(6,729.0)
Total stockholders' equity attributable to Principal Financial Group, Inc.	11,390.0	12,849.3
Total liabilities and stockholders' equity	243,036.1	253,941.2
Principal Financial Group, Inc. Parent Only
Assets
Fixed maturities, available-for-sale	211.5
Fixed maturities, trading	308.9	351.8
Cash and cash equivalents	334.9	842.8	$ 882.6	$ 578.7
Other investments	10.7	10.2
Income taxes receivable	16.9	44.2
Deferred income tax assets	331.0	329.4
Amounts receivable from subsidiaries	5.2	4.4
Other assets	23.3	12.8
Investment in unconsolidated entities	13,862.4	15,063.2
Total assets	15,104.8	16,658.8
Liabilities
Long-term debt	3,129.8	3,128.1
Accrued investment payable	23.1	23.1
Pension liability	557.9	658.3
Other liabilities	4.0
Total liabilities	3,714.8	3,809.5
Stockholders' equity
Common stock, par value $.01 per share - 2,500.0 million shares authorized, 476.7 million and 474.1 million shares issued, and 279.5 million and 289.0 million shares outstanding in 2018 and 2017	4.8	4.7
Additional paid-in capital	10,060.7	9,925.2
Retained earnings (accumulated deficit)	10,290.2	9,482.9
Accumulated other comprehensive income (loss)	(1,565.1)	165.5
Treasury stock, at cost (197.2 million and 185.1 million shares in 2018 and 2017)	(7,400.6)	(6,729.0)
Total stockholders' equity attributable to Principal Financial Group, Inc.	11,390.0	12,849.3
Total liabilities and stockholders' equity	$ 15,104.8	$ 16,658.8