Schedule II - Condensed Financial Information of Registrant (Parent Only) - Statements of Financial Position - Parenthetical (Details) - $ / shares shares in Millions	Dec. 31, 2017	Dec. 31, 2016
Condensed Financial Statements, Captions [Line Items]
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, authorized (in shares)	2,500.0	2,500.0
Common stock, issued (in shares)	474.1	469.2
Common stock, outstanding (in shares)	289.0	287.7
Treasury stock (in shares)	185.1	181.5
Principal Financial Group, Inc. Parent Only
Condensed Financial Statements, Captions [Line Items]
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, authorized (in shares)	2,500.0	2,500.0
Common stock, issued (in shares)	474.1	469.2
Common stock, outstanding (in shares)	289.0	287.7
Treasury stock (in shares)	185.1	181.5