Stockholders' Equity (Details 3) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
Change in accumulated other comprehensive income (loss) rollforward
Accumulated other comprehensive income (loss), balance at beginning of period	$ 640.3
Accumulated other comprehensive income (loss), balance at end of period	(17.7)
Change in redeemable noncontrolling interest rollforward
Redeemable noncontrolling interest, balance at beginning of period	60.4	22.2
Net income (loss) attributable to redeemable noncontrolling interest	9.0	1.6
Reclassification from stockholders' equity	173.9
Redeemable noncontrolling interest assumed related to acquisition		37.6
Contributions from redeemable noncontrolling interest	1.2
Distributions to redeemable noncontrolling interest	(9.9)	(1.3)
Purchase of subsidiary shares from redeemable noncontrolling interest	(2.4)
Change in redemption value of redeemable noncontrolling interest	5.6
Foreign currency translation adjustment, redeemable noncontrolling interest	(3.3)	0.9
Redeemable noncontrolling interest, balance at end of period	234.5	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	1,418.3	860.7
Other comprehensive income (loss) during the period, net of adjustments	(567.8)	495.8
Amounts reclassified from accumulated other comprehensive income (loss)	44.1	62.2
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.	(523.7)	558.0
Accumulated other comprehensive income (loss), balance at end of period	894.6	1,418.7
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	(173.9)	(167.2)
Other comprehensive income (loss) during the period, net of adjustments	(13.1)	(14.8)
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.	(13.1)	(14.8)
Accumulated other comprehensive income (loss), balance at end of period	(187.0)	(182.0)
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	(8.7)	34.9
Other comprehensive income (loss) during the period, net of adjustments	(4.2)	20.7
Amounts reclassified from accumulated other comprehensive income (loss)	0.8	6.5
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.	(3.4)	27.2
Accumulated other comprehensive income (loss), balance at end of period	(12.1)	62.1
Foreign currency translation adjustment
Change in accumulated other comprehensive income (loss) rollforward
Accumulated other comprehensive income (loss), balance at beginning of period	(106.9)	(95.9)
Other comprehensive income (loss) during the period, net of adjustments	(159.2)	20.5
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.	(159.2)	20.5
Accumulated other comprehensive income (loss), balance at end of period	(266.1)	(75.4)
Unrecognized postretirement benefit obligations
Change in accumulated other comprehensive income (loss) rollforward
Accumulated other comprehensive income (loss), balance at beginning of period	(488.5)	(361.1)
Amounts reclassified from accumulated other comprehensive income (loss)	41.4	26.2
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.	41.4	26.2
Accumulated other comprehensive income (loss), balance at end of period	(447.1)	(334.9)
Accumulated other comprehensive income (loss)
Change in accumulated other comprehensive income (loss) rollforward
Accumulated other comprehensive income (loss), balance at beginning of period	640.3	271.4
Other comprehensive income (loss) during the period, net of adjustments	(744.3)	522.2
Amounts reclassified from accumulated other comprehensive income (loss)	86.3	94.9
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.	(658.0)	617.1
Accumulated other comprehensive income (loss), balance at end of period	$ (17.7)	$ 888.5