Schedule II - Condensed Financial Information of Registrant (Parent Only) - Statements of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating activities
Net income (loss)	$ 491.5	$ 235.9	$ 416.1	$ 285.0	$ 315.3	$ 284.6	$ 392.1	$ 452.1	$ 1,428.5	$ 1,444.1	$ 1,553.7
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Net realized capital (gains) losses									(302.6)	52.8	75.4
Stock-based compensation									87.6	82.6	77.4
Changes in:
Net cash flows for trading securities and equity securities with operating intent									144.2	(53.8)	(133.7)
Current and deferred income taxes (benefits)									442.5	211.2	274.7
Other									110.4	11.1	(35.7)
Net cash provided by (used in) operating activities									3,738.6	5,493.2	5,156.5
Investing activities
Fixed maturities available-for-sale and equity securities with intent to hold: Purchases									(15,713.4)	(14,137.1)	(13,909.8)
Fixed maturities available-for-sale and equity securities with intent to hold: Maturities									8,819.5	7,064.2	6,217.6
Net (purchases) sales of property and equipment									(108.8)	(132.4)	(92.3)
Net change in other investments									(66.4)	(489.1)	(302.7)
Net cash provided by (used in) investing activities									(5,025.8)	(7,688.5)	(5,702.5)
Financing activities
Issuance of common stock									42.8	37.7	64.0
Acquisition of treasury stock									(307.0)	(281.0)	(671.6)
Dividends to common stockholders									(614.5)	(606.0)	(598.6)
Principal repayments of long-term debt									(65.8)	(32.2)	(1.3)
Issuance of long-term debt									608.9	504.9	80.8
Net cash provided by (used in) financing activities									1,621.1	1,733.7	1,052.7
Net increase (decrease) in cash and cash equivalents									333.9	(461.6)	506.7
Cash and cash equivalents at beginning of period				2,515.9				2,977.5	2,515.9	2,977.5	2,470.8
Cash and cash equivalents at end of period	2,849.8				2,515.9				2,849.8	2,515.9	2,977.5
Principal Financial Group, Inc. parent only | Legal Entities
Operating activities
Net income (loss)									1,395.8	1,394.2	1,546.5
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Net realized capital (gains) losses									(7.0)	(12.3)	8.2
Stock-based compensation									1.4	1.3	2.9
Equity in the net (income) loss of subsidiaries									(1,529.4)	(1,529.2)	(1,658.3)
Changes in:
Net cash flows for trading securities and equity securities with operating intent									88.4	48.7	32.4
Current and deferred income taxes (benefits)									(13.5)	(31.0)	15.7
Other									49.1	135.4	(79.8)
Net cash provided by (used in) operating activities									(15.2)	7.1	(132.4)
Investing activities
Fixed maturities available-for-sale and equity securities with intent to hold: Purchases									(736.5)	(251.5)	(210.9)
Fixed maturities available-for-sale and equity securities with intent to hold: Maturities									193.7	200.0	2.3
Net (purchases) sales of property and equipment									(0.1)	(0.1)	(0.1)
Net change in other investments									(50.0)	(34.0)	(2.2)
Dividends and returns of capital received from unconsolidated entities									799.1	494.2	1,041.6
Net cash provided by (used in) investing activities									206.2	408.6	830.7
Financing activities
Issuance of common stock									42.8	37.7	64.0
Acquisition of treasury stock									(307.0)	(281.0)	(671.6)
Dividends to common stockholders									(614.5)	(606.0)	(598.6)
Issuance of long-term debt									595.2	493.6
Net cash provided by (used in) financing activities									(283.5)	(355.7)	(1,206.2)
Net increase (decrease) in cash and cash equivalents									(92.5)	60.0	(507.9)
Cash and cash equivalents at beginning of period				$ 394.9				$ 334.9	394.9	334.9	842.8
Cash and cash equivalents at end of period	$ 302.4				$ 394.9				$ 302.4	$ 394.9	$ 334.9