Stockholders' Equity (Details 3) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Change in accumulated other comprehensive income (loss) rollforward
Accumulated other comprehensive income (loss), balance at beginning of period	$ 50.4
Purchase of subsidiary shares from noncontrolling interest	(8.7)	(25.3)
Accumulated other comprehensive income (loss), balance at end of period	28.0
Change in redeemable noncontrolling interest rollforward
Redeemable noncontrolling interest, balance at beginning of period	58.0	247.2
Net income (loss) attributable to redeemable noncontrolling interest	2.9	2.0
Contributions from redeemable noncontrolling interest	10.1
Distributions to redeemable noncontrolling interest	(2.0)	(4.9)
Change in redemption value of redeemable noncontrolling interest	3.7	34.3
Foreign currency translation adjustment, redeemable noncontrolling interest	(7.7)	1.6
Redeemable noncontrolling interest, balance at end of period	65.0	280.2
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	1,202.8	878.1
Other comprehensive income (loss) during the period, net of adjustments	66.0	207.6
Amounts reclassified from accumulated other comprehensive income (loss)	(8.9)	(15.4)
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.	57.1	192.2
Accumulated other comprehensive income (loss), balance at end of period	1,259.9	1,070.3
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	(105.1)	(167.0)
Amounts reclassified from accumulated other comprehensive income (loss)	13.5	17.5
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.	13.5	17.5
Accumulated other comprehensive income (loss), balance at end of period	(91.6)	(149.5)
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	50.6	(10.5)
Other comprehensive income (loss) during the period, net of adjustments	34.5	2.8
Amounts reclassified from accumulated other comprehensive income (loss)	(6.7)	6.2
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.	27.8	9.0
Accumulated other comprehensive income (loss), balance at end of period	78.4	(1.5)
Foreign currency translation adjustment
Change in accumulated other comprehensive income (loss) rollforward
Accumulated other comprehensive income (loss), balance at beginning of period	(686.8)	(361.5)
Other comprehensive income (loss) during the period, net of adjustments	(126.3)	(43.6)
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.	(126.3)	(43.6)
Purchase of subsidiary shares from noncontrolling interest	(6.5)
Accumulated other comprehensive income (loss), balance at end of period	(819.6)	(405.1)
Unrecognized postretirement benefit obligations
Change in accumulated other comprehensive income (loss) rollforward
Accumulated other comprehensive income (loss), balance at beginning of period	(411.1)	(155.9)
Amounts reclassified from accumulated other comprehensive income (loss)	12.0	3.5
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.	12.0	3.5
Accumulated other comprehensive income (loss), balance at end of period	(399.1)	(152.4)
Accumulated other comprehensive income (loss)
Change in accumulated other comprehensive income (loss) rollforward
Accumulated other comprehensive income (loss), balance at beginning of period	50.4	183.2
Other comprehensive income (loss) during the period, net of adjustments	(25.8)	166.8
Amounts reclassified from accumulated other comprehensive income (loss)	9.9	11.8
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.	(15.9)	178.6
Purchase of subsidiary shares from noncontrolling interest	(6.5)
Accumulated other comprehensive income (loss), balance at end of period	$ 28.0	$ 361.8