Nature of Operations and Significant Accounting Policies - Separate Accounts (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Separate Accounts
Separate account that primarily includes shares of Principal Financial Group, Inc. stock that were allocated and issued to eligible participants of qualified employee benefit plans as part of the 2001 demutualization	$ 162.9	$ 158.4

This item represents the value of a separate account that primarily includes shares of Principal Financial Group, Inc. stock that were allocated and issued to eligible participants of qualified employee benefit plans administered as part of the policy credits issued under the 2001 demutualization.

No definition available.

Name:	pfg_SeparateAccountWithSharesOfPrincipalFinancialGroupStock
Namespace Prefix:	pfg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant