Stockholders' Equity (Details 3) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011
Change in accumulated other comprehensive income (loss) rollforward
Accumulated other comprehensive income (loss), balance at beginning of period	$ 258.0
Accumulated other comprehensive income (loss), balance at end of period	880.8
Change in redeemable noncontrolling interest rollforward
Redeemable noncontrolling interest, balance at beginning of period	22.2
Net income (loss) attributable to redeemable noncontrolling interest	1.6
Redeemable noncontrolling interest assumed related to acquisition	37.6
Distributions to redeemable noncontrolling interest	(1.3)
Foreign currency translation adjustment, redeemable noncontrolling interest	0.9
Redeemable noncontrolling interest, balance at end of period	61.0
Net unrealized gains (losses) on available-for-sale securities
Change in accumulated other comprehensive income (loss) rollforward
Accumulated other comprehensive income (loss), balance at beginning of period	860.7	652.1
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.	558.0	329.5
Accumulated other comprehensive income (loss), balance at end of period	1,418.7	981.6
Noncredit component of impairment losses on fixed maturities available-for-sale
Change in accumulated other comprehensive income (loss) rollforward
Accumulated other comprehensive income (loss), balance at beginning of period	(167.2)	(198.2)
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.	(14.8)	45.2
Accumulated other comprehensive income (loss), balance at end of period	(182.0)	(153.0)
Net unrealized gains (losses) on derivative instruments
Change in accumulated other comprehensive income (loss) rollforward
Accumulated other comprehensive income (loss), balance at beginning of period	34.9	11.3
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.	27.2	19.5
Accumulated other comprehensive income (loss), balance at end of period	62.1	30.8
Foreign currency translation adjustment
Change in accumulated other comprehensive income (loss) rollforward
Accumulated other comprehensive income (loss), balance at beginning of period	(109.3)	29.7
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.	26.2	(64.6)
Accumulated other comprehensive income (loss), balance at end of period	(83.1)	(34.9)
Unrecognized postretirement benefit obligations
Change in accumulated other comprehensive income (loss) rollforward
Accumulated other comprehensive income (loss), balance at beginning of period	(361.1)	(188.2)
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.	26.2	(174.2)
Accumulated other comprehensive income (loss), balance at end of period	(334.9)	(362.4)
Accumulated other comprehensive income (loss)
Change in accumulated other comprehensive income (loss) rollforward
Accumulated other comprehensive income (loss), balance at beginning of period	258.0	306.7
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.	622.8	155.4
Accumulated other comprehensive income (loss), balance at end of period	$ 880.8	$ 462.1