Goodwill and Other Intangible Assets - Changes in Goodwill (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Changes in Goodwill
Beginning balance, Goodwill	$ 1,020.8	$ 1,009.0
Foreign currency, Goodwill	48.0	11.8
Ending balance, Goodwill	1,068.8	1,020.8
Retirement and Income Solutions
Changes in Goodwill
Beginning balance, Goodwill	57.4	57.4
Ending balance, Goodwill	57.4	57.4
Principal Global Investors
Changes in Goodwill
Beginning balance, Goodwill	242.5	252.1
Foreign currency, Goodwill	4.8	(9.6)
Ending balance, Goodwill	247.3	242.5
Principal International
Changes in Goodwill
Beginning balance, Goodwill	664.3	642.9
Foreign currency, Goodwill	43.2	21.4
Ending balance, Goodwill	707.5	664.3
U.S. Insurance Solutions
Changes in Goodwill
Beginning balance, Goodwill	56.6	56.6
Ending balance, Goodwill	$ 56.6	$ 56.6