Consolidated Statements of Stockholders' Equity (Parenthetical) (USD $) In Millions, unless otherwise specified	9 Months Ended
Sep. 30, 2012
Consolidated Statements of Stockholders' Equity
Net income (loss) attributable to redeemable noncontrolling interest	$ 1.6
Other comprehensive income (loss) attributable to redeemable noncontrolling interest	$ 0.9

Amount of Other Comprehensive Income (Loss) attributable to redeemable noncontrolling interest.

No definition available.

Name:	pfg_OtherComprehensiveIncomeLossAttributableToRedeemableNoncontrollingInterest
Namespace Prefix:	pfg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration