Stockholders' Equity (Details 3) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Change in accumulated other comprehensive income (loss) rollforward
Balances	$ 10,184.0
Purchase of subsidiary shares from noncontrolling interest	(16.5)	$ (41.9)
Balances	9,620.3
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	1.5	8.3
Contributions from redeemable noncontrolling interest	49.3	0.1
Distributions to redeemable noncontrolling interest	(10.7)	(14.4)
Purchase of subsidiary shares from redeemable noncontrolling interest	(6.0)	(216.8)
Change in redemption value of redeemable noncontrolling interest	3.6	33.2
Foreign currency translation adjustment, redeemable noncontrolling interest	(17.7)	(1.5)
Redeemable noncontrolling interest, balance at end of period	78.0	56.1
Accumulated other comprehensive income (loss)
Change in accumulated other comprehensive income (loss) rollforward
Balances	50.4	183.2
Other comprehensive income (loss) during the period, net of adjustments	(602.3)	172.0
Amounts reclassified from accumulated other comprehensive income (loss)	37.8	24.9
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.	(564.5)	196.9
Purchase of subsidiary shares from noncontrolling interest	(10.3)
Balances	(524.4)	380.1
Net unrealized gains (losses) on available-for-sale securities
Change in accumulated other comprehensive income (loss) rollforward
Balances	1,202.8	878.1
Other comprehensive income (loss) during the period, net of adjustments	(219.8)	335.2
Amounts reclassified from accumulated other comprehensive income (loss)	(1.2)	(38.6)
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.	(221.0)	296.6
Balances	981.8	1,174.7
Noncredit component of impairment losses on fixed maturities available-for-sale
Change in accumulated other comprehensive income (loss) rollforward
Balances	(105.1)	(167.0)
Amounts reclassified from accumulated other comprehensive income (loss)	17.9	48.9
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.	17.9	48.9
Balances	(87.2)	(118.1)
Net unrealized gains (losses) on derivative instruments
Change in accumulated other comprehensive income (loss) rollforward
Balances	50.6	(10.5)
Other comprehensive income (loss) during the period, net of adjustments	36.8	38.4
Amounts reclassified from accumulated other comprehensive income (loss)	(14.8)	3.9
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.	22.0	42.3
Balances	72.6	31.8
Foreign currency translation adjustment
Change in accumulated other comprehensive income (loss) rollforward
Balances	(686.8)	(361.5)
Other comprehensive income (loss) during the period, net of adjustments	(419.3)	(201.6)
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.	(419.3)	(201.6)
Purchase of subsidiary shares from noncontrolling interest	(10.3)
Balances	(1,116.4)	(563.1)
Unrecognized postretirement benefit obligation
Change in accumulated other comprehensive income (loss) rollforward
Balances	(411.1)	(155.9)
Amounts reclassified from accumulated other comprehensive income (loss)	35.9	10.7
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.	35.9	10.7
Balances	$ (375.2)	$ (145.2)