Stockholders' Equity - AOCI and Noncontrolling Interest (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in accumulated other comprehensive income (loss) rollforward
Balances		$ 10,227.3
Purchase of subsidiary shares from noncontrolling interest	[1]	(8.9)	$ 5.8	$ (16.5)
Balances		12,849.3	10,227.3
Change in redeemable noncontrolling interest rollforward
Redeemable noncontrolling interest, balance at beginning of period		97.5	85.7	58.0
Net income (loss) attributable to redeemable noncontrolling interest		7.0	16.8	4.9
Redeemable noncontrolling interest of newly consolidated entities			179.5
Redeemable noncontrolling interest of deconsolidated entities		(61.1)	(261.5)
Contributions from redeemable noncontrolling interest		94.1	135.1	56.1
Distributions to redeemable noncontrolling interest		(39.0)	(57.4)	(15.1)
Purchase of subsidiary shares from redeemable noncontrolling interest		(4.4)	(8.2)	(6.0)
Change in redemption value of redeemable noncontrolling interest		5.9	4.2	6.6
Other comprehensive income (loss) attributable to redeemable noncontrolling interest		1.3	3.3	(18.8)
Redeemable noncontrolling interest, balance at end of period		101.3	97.5	85.7
Accumulated other comprehensive income (loss)
Change in accumulated other comprehensive income (loss) rollforward
Balances		(675.2)	(882.5)	50.4
Other comprehensive income (loss) during the period, net of adjustments		768.3	166.1	(974.9)
Amounts reclassified from accumulated other comprehensive income (loss)		72.4	50.5	52.3
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.		840.7	216.6	(922.6)
Purchase of subsidiary shares from noncontrolling interest	[1]		(9.3)	(10.3)
Balances		165.5	(675.2)	(882.5)
Net unrealized gains (losses) on available-for-sale securities
Change in accumulated other comprehensive income (loss) rollforward
Balances		831.2	732.1	1,202.8
Other comprehensive income (loss) during the period, net of adjustments		595.2	54.7	(480.4)
Amounts reclassified from accumulated other comprehensive income (loss)		44.3	44.4	9.7
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.		639.5	99.1	(470.7)
Balances		1,470.7	831.2	732.1
Noncredit component of impairment losses on fixed maturities available-for-sale
Change in accumulated other comprehensive income (loss) rollforward
Balances		(89.5)	(86.0)	(105.1)
Other comprehensive income (loss) during the period, net of adjustments			(3.5)
Amounts reclassified from accumulated other comprehensive income (loss)		31.2		19.1
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.		31.2	(3.5)	19.1
Balances		(58.3)	(89.5)	(86.0)
Net unrealized gains (losses) on derivative instruments
Change in accumulated other comprehensive income (loss) rollforward
Balances		85.3	69.8	50.6
Other comprehensive income (loss) during the period, net of adjustments		(21.8)	37.8	43.6
Amounts reclassified from accumulated other comprehensive income (loss)		(20.8)	(22.3)	(24.4)
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.		(42.6)	15.5	19.2
Balances		42.7	85.3	69.8
Foreign currency translation adjustment
Change in accumulated other comprehensive income (loss) rollforward
Balances		(1,093.8)	(1,148.2)	(686.8)
Other comprehensive income (loss) during the period, net of adjustments		174.7	63.7	(451.1)
Amounts reclassified from accumulated other comprehensive income (loss)		0.6
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.		175.3	63.7	(451.1)
Purchase of subsidiary shares from noncontrolling interest			(9.3)	(10.3)
Balances		(918.5)	(1,093.8)	(1,148.2)
Unrecognized postretirement benefit obligation
Change in accumulated other comprehensive income (loss) rollforward
Balances		(408.4)	(450.2)	(411.1)
Other comprehensive income (loss) during the period, net of adjustments		20.2	13.4	(87.0)
Amounts reclassified from accumulated other comprehensive income (loss)		17.1	28.4	47.9
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.		37.3	41.8	(39.1)
Balances		$ (371.1)	$ (408.4)	$ (450.2)

[1] Excludes amounts attributable to redeemable noncontrolling interest. See Note 13, Stockholders’ Equity, for further details.