Schedule II - Condensed Financial Information of Registrant (Parent Only) - Financial Statement Footnotes (Details) - Principal Financial Group, Inc. Parent Only - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Dividends and Returns of Capital Received from (Contributed to) Unconsolidated Entity
Dividends and returns of capital received from (contributed to) unconsolidated entities	$ 1,002.5	$ 1,295.3	$ 685.5
Supplemental Disclosures of Non-Cash Investing Activity
Non-cash increase in investment in subsidiaries		74.6
Nonqualified benefit plans
Condensed Financial Statements, Captions [Line Items]
Assets held in Rabbi trusts	643.3	570.9
Liabilities held in Rabbi trusts	$ 524.7	$ 477.3