Goodwill and Other Intangible Assets - Changes in Goodwill (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Changes in Goodwill
Beginning balance, Goodwill	$ 1,068.8	$ 1,020.8
Goodwill from acquisitions	113.9
Goodwill disposed	(12.0)
Foreign currency, Goodwill	(70.7)	48.0
Ending balance, Goodwill	1,100.0	1,068.8
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	247.3	242.5
Goodwill from acquisitions	65.0
Foreign currency, Goodwill	(5.0)	4.8
Ending balance, Goodwill	307.3	247.3
Principal International
Changes in Goodwill
Beginning balance, Goodwill	707.5	664.3
Goodwill from acquisitions	46.8
Goodwill disposed	(12.0)
Foreign currency, Goodwill	(65.7)	43.2
Ending balance, Goodwill	676.6	707.5
U.S. Insurance Solutions
Changes in Goodwill
Beginning balance, Goodwill	56.6	56.6
Ending balance, Goodwill	56.6	$ 56.6
Corporate
Changes in Goodwill
Goodwill from acquisitions	2.1
Ending balance, Goodwill	$ 2.1

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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Amount of divestiture of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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