Condensed Consolidating Financial Information - Shelf Registration Guarantor, Statements of Cash Flows (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Operating activities
Net cash provided by (used in) operating activities	$ 3,628.7	$ 2,927.3
Investing activities
Available-for-sale securities: Purchases	(10,425.7)	(10,590.0)
Available-for-sale securities: Sales	1,011.2	1,138.1
Available-for-sale securities: Maturities	6,577.2	5,836.0
Mortgage loans acquired or originated	(2,036.5)	(2,116.3)
Mortgage loans sold or repaid	1,297.4	1,440.8
Real estate acquired	(113.6)	(90.2)
Real estate sold	457.1	35.5
Net (purchases) sales of property and equipment	(133.3)	(109.1)
Net change in other investments	48.5	(42.4)
Net cash provided by (used in) investing activities	(3,317.7)	(4,497.6)
Financing activities
Issuance of common stock	143.1	25.7
Acquisition of treasury stock	(218.1)	(245.2)
Proceeds from financing element derivatives	0.1	0.3
Payments for financing element derivatives	(59.5)	(67.3)
Excess tax benefits from share-based payment arrangements		5.9
Purchase of subsidiary shares from noncontrolling interest	(13.3)	(2.4)
Dividends to common stockholders	(398.5)	(341.1)
Issuance of long-term debt		11.0
Principal repayments of long-term debt	(56.2)
Net proceeds from (repayments of) short-term borrowings	4.9	(143.4)
Investment contract deposits	8,244.4	8,108.5
Investment contract withdrawals	(7,963.1)	(6,201.4)
Net increase (decrease) in banking operation deposits	79.8	69.6
Other	0.8	3.5
Net cash provided by (used in) financing activities	(235.6)	1,223.7
Net increase (decrease) in cash and cash equivalents	75.4	(346.6)
Cash and cash equivalents at beginning of period	2,719.6	2,564.8
Cash and cash equivalents at end of period	2,795.0	2,218.2
Principal Financial Group, Inc. Parent Only
Operating activities
Net cash provided by (used in) operating activities	(36.7)	(45.0)
Investing activities
Dividends and returns of capital received from (contributed to) unconsolidated entities	250.1	795.2
Net change in other investments	(4.4)	(6.8)
Net cash provided by (used in) investing activities	245.7	788.4
Financing activities
Issuance of common stock	143.1	25.7
Acquisition of treasury stock	(218.1)	(245.2)
Excess tax benefits from share-based payment arrangements		0.2
Dividends to common stockholders	(398.5)	(341.1)
Net cash provided by (used in) financing activities	(473.5)	(560.4)
Net increase (decrease) in cash and cash equivalents	(264.5)	183.0
Cash and cash equivalents at beginning of period	882.6	578.7
Cash and cash equivalents at end of period	618.1	761.7
Principal Financial Services, Inc. Only
Operating activities
Net cash provided by (used in) operating activities	449.3	(2.5)
Investing activities
Dividends and returns of capital received from (contributed to) unconsolidated entities	(37.0)	886.0
Net change in other investments	(70.5)	220.3
Net cash provided by (used in) investing activities	(107.5)	1,106.3
Financing activities
Issuance of long-term debt	6.4	5.0
Principal repayments of long-term debt	(19.5)	(20.0)
Capital received from (dividends and capital paid to) parent	(250.1)	(795.2)
Net cash provided by (used in) financing activities	(263.2)	(810.2)
Net increase (decrease) in cash and cash equivalents	78.6	293.6
Cash and cash equivalents at beginning of period	1,203.4	730.5
Cash and cash equivalents at end of period	1,282.0	1,024.1
Principal Life Insurance Company and Other Subsidiaries Combined
Operating activities
Net cash provided by (used in) operating activities	3,721.0	2,849.7
Investing activities
Available-for-sale securities: Purchases	(10,425.7)	(10,590.0)
Available-for-sale securities: Sales	1,011.2	1,138.1
Available-for-sale securities: Maturities	6,577.2	5,836.0
Mortgage loans acquired or originated	(2,036.5)	(2,116.3)
Mortgage loans sold or repaid	1,297.4	1,440.8
Real estate acquired	(113.6)	(90.2)
Real estate sold	457.1	35.5
Net (purchases) sales of property and equipment	(133.3)	(109.1)
Net change in other investments	(106.6)	121.2
Net cash provided by (used in) investing activities	(3,472.8)	(4,334.0)
Financing activities
Proceeds from financing element derivatives	0.1	0.3
Payments for financing element derivatives	(59.5)	(67.3)
Excess tax benefits from share-based payment arrangements		5.7
Purchase of subsidiary shares from noncontrolling interest	(13.3)	(2.4)
Issuance of long-term debt		78.0
Principal repayments of long-term debt	(56.2)
Net proceeds from (repayments of) short-term borrowings	(71.6)	(166.0)
Capital received from (dividends and capital paid to) parent	37.0	(886.0)
Investment contract deposits	8,244.4	8,108.5
Investment contract withdrawals	(7,963.1)	(6,201.4)
Net increase (decrease) in banking operation deposits	79.8	69.6
Other	0.8	3.5
Net cash provided by (used in) financing activities	198.4	942.5
Net increase (decrease) in cash and cash equivalents	446.6	(541.8)
Cash and cash equivalents at beginning of period	2,114.8	2,413.3
Cash and cash equivalents at end of period	2,561.4	1,871.5
Eliminations, Shelf Registration Debt Guarantor
Operating activities
Net cash provided by (used in) operating activities	(504.9)	125.1
Investing activities
Dividends and returns of capital received from (contributed to) unconsolidated entities	(213.1)	(1,681.2)
Net change in other investments	230.0	(377.1)
Net cash provided by (used in) investing activities	16.9	(2,058.3)
Financing activities
Issuance of long-term debt	(6.4)	(72.0)
Principal repayments of long-term debt	19.5	20.0
Net proceeds from (repayments of) short-term borrowings	76.5	22.6
Capital received from (dividends and capital paid to) parent	213.1	1,681.2
Net cash provided by (used in) financing activities	302.7	1,651.8
Net increase (decrease) in cash and cash equivalents	(185.3)	(281.4)
Cash and cash equivalents at beginning of period	(1,481.2)	(1,157.7)
Cash and cash equivalents at end of period	$ (1,666.5)	$ (1,439.1)