Stockholders' Equity - AOCI and Noncontrolling Interest (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Change in accumulated other comprehensive income (loss) rollforward
Balances	$ 9,311.6
Accumulated other comprehensive income (loss), balance at beginning of period	(882.5)
Purchase of subsidiary shares from noncontrolling interest	5.8	$ (8.7)
Accumulated other comprehensive income (loss), balance at end of period	(356.1)
Balances	10,041.6
Change in redeemable noncontrolling interest rollforward
Redeemable noncontrolling interest, balance at beginning of period	85.7	58.0
Net income (loss) attributable to redeemable noncontrolling interest	(0.2)	2.9
Redeemable noncontrolling interest of newly consolidated entities	179.5
Contributions from redeemable noncontrolling interest	22.1	10.1
Distributions to redeemable noncontrolling interest	(17.1)	(2.0)
Purchase of subsidiary shares from redeemable noncontrolling interest	(8.1)
Change in redemption value of redeemable noncontrolling interest	(3.1)	3.7
Foreign currency translation adjustment, redeemable noncontrolling interest	2.1	(7.7)
Redeemable noncontrolling interest, balance at end of period	260.9	65.0
Accumulated other comprehensive income (loss)
Change in accumulated other comprehensive income (loss) rollforward
Balances	(882.5)	50.4
Other comprehensive income (loss) during the period, net of adjustments	493.9	(25.8)
Amounts reclassified from accumulated other comprehensive income (loss)	41.8	9.9
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.	535.7	(15.9)
Purchase of subsidiary shares from noncontrolling interest	(9.3)	(6.5)
Balances	(356.1)	28.0
Net unrealized gains (losses) on available-for-sale securities
Change in accumulated other comprehensive income (loss) rollforward
Balances	732.1	1,202.8
Other comprehensive income (loss) during the period, net of adjustments	368.5	66.0
Amounts reclassified from accumulated other comprehensive income (loss)	35.9	(8.9)
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.	404.4	57.1
Balances	1,136.5	1,259.9
Noncredit component of impairment losses on fixed maturities available-for-sale
Change in accumulated other comprehensive income (loss) rollforward
Balances	(86.0)	(105.1)
Other comprehensive income (loss) during the period, net of adjustments	(4.2)
Amounts reclassified from accumulated other comprehensive income (loss)		13.5
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.	(4.2)	13.5
Balances	(90.2)	(91.6)
Net unrealized gains (losses) on derivative instruments
Change in accumulated other comprehensive income (loss) rollforward
Balances	69.8	50.6
Other comprehensive income (loss) during the period, net of adjustments	3.4	34.5
Amounts reclassified from accumulated other comprehensive income (loss)	(2.1)	(6.7)
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.	1.3	27.8
Balances	71.1	78.4
Foreign currency translation adjustment
Change in accumulated other comprehensive income (loss) rollforward
Balances	(1,148.2)	(686.8)
Other comprehensive income (loss) during the period, net of adjustments	126.2	(126.3)
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.	126.2	(126.3)
Purchase of subsidiary shares from noncontrolling interest	(9.3)	(6.5)
Balances	(1,031.3)	(819.6)
Unrecognized postretirement benefit obligation
Change in accumulated other comprehensive income (loss) rollforward
Balances	(450.2)	(411.1)
Amounts reclassified from accumulated other comprehensive income (loss)	8.0	12.0
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.	8.0	12.0
Balances	$ (442.2)	$ (399.1)