Stockholders' Equity - AOCI and Noncontrolling Interest (Details) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016
Change in accumulated other comprehensive income (loss) rollforward
Balances		$ 10,227.3
Purchase of subsidiary shares from noncontrolling interest	[1]	(8.9)	$ 5.8
Balances		11,996.4
Change in redeemable noncontrolling interest rollforward
Redeemable noncontrolling interest, balance at beginning of period		97.5	85.7
Net income (loss) attributable to redeemable noncontrolling interest		6.9	20.8
Redeemable noncontrolling interest of newly consolidated entities			179.5
Redeemable noncontrolling interest of deconsolidated entities		(1.4)	(181.2)
Contributions from redeemable noncontrolling interest		54.5	132.5
Distributions to redeemable noncontrolling interest		(28.2)	(46.9)
Purchase of subsidiary shares from redeemable noncontrolling interest		(4.4)	(8.2)
Change in redemption value of redeemable noncontrolling interest		2.8	4.1
Other comprehensive income (loss) attributable to redeemable noncontrolling interest		1.4	3.6
Redeemable noncontrolling interest, balance at end of period		129.1	189.9
Accumulated other comprehensive income (loss)
Change in accumulated other comprehensive income (loss) rollforward
Balances		(675.2)	(882.5)
Other comprehensive income (loss) during the period, net of adjustments		669.3	954.2
Amounts reclassified from accumulated other comprehensive income (loss)		52.9	29.4
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.		722.2	983.6
Purchase of subsidiary shares from noncontrolling interest	[1]		(9.3)
Balances		47.0	91.8
Net unrealized gains (losses) on available-for-sale securities
Change in accumulated other comprehensive income (loss) rollforward
Balances		831.2	732.1
Other comprehensive income (loss) during the period, net of adjustments		536.2	816.5
Amounts reclassified from accumulated other comprehensive income (loss)		32.9	11.4
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.		569.1	827.9
Balances		1,400.3	1,560.0
Noncredit component of impairment losses on fixed maturities available-for-sale
Change in accumulated other comprehensive income (loss) rollforward
Balances		(89.5)	(86.0)
Other comprehensive income (loss) during the period, net of adjustments			(6.9)
Amounts reclassified from accumulated other comprehensive income (loss)		21.0
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.		21.0	(6.9)
Balances		(68.5)	(92.9)
Net unrealized gains (losses) on derivative instruments
Change in accumulated other comprehensive income (loss) rollforward
Balances		85.3	69.8
Other comprehensive income (loss) during the period, net of adjustments		(15.5)	22.2
Amounts reclassified from accumulated other comprehensive income (loss)		(13.8)	(6.0)
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.		(29.3)	16.2
Balances		56.0	86.0
Foreign currency translation adjustment
Change in accumulated other comprehensive income (loss) rollforward
Balances		(1,093.8)	(1,148.2)
Other comprehensive income (loss) during the period, net of adjustments		148.6	122.4
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.		148.6	122.4
Purchase of subsidiary shares from noncontrolling interest			(9.3)
Balances		(945.2)	(1,035.1)
Unrecognized postretirement benefit obligation
Change in accumulated other comprehensive income (loss) rollforward
Balances		(408.4)	(450.2)
Amounts reclassified from accumulated other comprehensive income (loss)		12.8	24.0
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.		12.8	24.0
Balances		$ (395.6)	$ (426.2)

[1] Excludes amounts attributable to redeemable noncontrolling interest. See Note 9, Stockholders’ Equity, for further details.