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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating activities
Net income (loss)	$ 842.9	$ 813.2	$ 314.8	$ 353.6	$ 338.2	$ 327.4	$ 327.0	$ 369.2	$ 2,324.5	$ 1,361.8	$ 1,253.2
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Net realized capital (gains) losses									(524.2)	(171.1)	51.1
Stock-based compensation									82.7	84.4	84.5
Changes in:
Net cash flows for trading securities									(332.2)	166.5	(201.9)
Current and deferred income taxes (benefits)									(164.4)	45.8	(66.5)
Other									17.0	(72.1)	(102.3)
Net cash provided by (used in) operating activities									4,188.0	3,857.8	4,377.1
Investing activities
Net (purchases) sales of property and equipment									(164.8)	(154.9)	(136.4)
Net change in other investments									(70.8)	26.0	(307.7)
Net cash provided by (used in) investing activities									(4,052.8)	(5,153.9)	(3,167.6)
Financing activities
Issuance of common stock									162.5	37.8	76.1
Acquisition of treasury stock									(220.4)	(277.3)	(300.6)
Excess tax benefits from share-based payment arrangements										12.0	15.7
Dividends to common stockholders									(540.0)	(464.9)	(441.0)
Dividends to preferred stockholders											(16.5)
Preferred stock redemption											(550.0)
Principal repayments of long-term debt									(56.5)	(799.3)	(52.6)
Issuance of long-term debt									2.8	656.1	804.9
Net cash provided by (used in) financing activities									(384.0)	1,450.9	(508.6)
Net increase (decrease) in cash and cash equivalents									(248.8)	154.8	700.9
Cash and cash equivalents at beginning of period				2,719.6				2,564.8	2,719.6	2,564.8	1,863.9
Cash and cash equivalents at end of period	2,470.8				2,719.6				2,470.8	2,719.6	2,564.8
Principal Financial Group, Inc. Parent Only
Operating activities
Net income (loss)									2,310.4	1,316.5	1,234.0
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Net realized capital (gains) losses									0.6
Stock-based compensation									3.0	3.6	3.9
Equity in the net (income) loss of subsidiaries									(2,594.7)	(1,490.5)	(1,336.6)
Changes in:
Net cash flows for trading securities									(352.6)
Current and deferred income taxes (benefits)									163.8	(60.3)	(20.9)
Other									26.3	42.1	34.5
Net cash provided by (used in) operating activities									(443.2)	(188.6)	(85.1)
Investing activities
Net (purchases) sales of property and equipment									(0.1)	(0.1)
Net change in other investments									(1.1)	1.3	5.4
Dividends and returns of capital received from (contributed to) unconsolidated entities									1,002.5	1,295.3	685.5
Net cash provided by (used in) investing activities									1,001.3	1,296.5	690.9
Financing activities
Issuance of common stock									162.5	37.8	76.1
Acquisition of treasury stock									(220.4)	(277.3)	(300.6)
Excess tax benefits from share-based payment arrangements										0.7	0.7
Dividends to common stockholders									(540.0)	(464.9)	(441.0)
Dividends to preferred stockholders											(16.5)
Preferred stock redemption											(550.0)
Principal repayments of long-term debt										(744.5)
Issuance of long-term debt										644.2	791.8
Net cash provided by (used in) financing activities									(597.9)	(804.0)	(439.5)
Net increase (decrease) in cash and cash equivalents									(39.8)	303.9	166.3
Cash and cash equivalents at beginning of period				$ 882.6				$ 578.7	882.6	578.7	412.4
Cash and cash equivalents at end of period	$ 842.8				$ 882.6				$ 842.8	$ 882.6	$ 578.7