Quarterly Results of Operations (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Results of Operations (Unaudited)
Total revenues	$ 3,770.6	$ 4,348.1	$ 3,234.9	$ 2,883.6	$ 3,237.7	$ 4,627.8	$ 3,178.3	$ 3,049.4	$ 14,237.2	$ 14,093.2	$ 12,394.1
Total expenses	3,527.7	3,779.0	2,719.5	2,426.6	2,922.9	3,470.0	2,812.7	2,635.4	12,452.8	11,841.0	10,802.4
Net income (loss)	231.7	460.0	459.5	402.5	842.9	813.2	314.8	353.6	1,553.7	2,324.5	1,361.8
Net income (loss) attributable to PFG	$ 236.5	$ 456.3	$ 456.6	$ 397.1	$ 841.8	$ 810.2	$ 309.5	$ 348.9	$ 1,546.5	$ 2,310.4	$ 1,316.5
Basic earnings per common share (in dollars per share)	$ 0.84	$ 1.60	$ 1.59	$ 1.37	$ 2.91	$ 2.80	$ 1.07	$ 1.21	$ 5.41	$ 8.00	$ 4.55
Diluted earnings per common share (in dollars per share)	$ 0.83	$ 1.59	$ 1.58	$ 1.36	$ 2.87	$ 2.76	$ 1.06	$ 1.19	$ 5.36	$ 7.88	$ 4.50