Condensed Consolidating Financial Information - Guarantor, Statements of Cash Flows (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Operating activities
Net cash provided by (used in) operating activities	$ 1,829.1	$ 2,125.6
Investing activities
Available-for-sale securities: Purchases	(7,236.9)	(7,476.9)
Available-for-sale securities: Sales	853.3	557.4
Available-for-sale securities: Maturities	4,621.5	3,667.9
Mortgage loans acquired or originated	(1,365.3)	(1,198.0)
Mortgage loans sold or repaid	843.1	910.4
Real estate acquired	(96.8)	(66.4)
Real estate sold	47.1	27.3
Net (purchases) sales of property and equipment	(104.1)	(74.7)
Net change in other investments	(66.2)	(21.0)
Net cash provided by (used in) investing activities	(2,504.3)	(3,674.0)
Financing activities
Issuance of common stock	113.3	11.2
Acquisition of treasury stock	(169.2)	(209.7)
Proceeds from financing element derivatives	0.1	0.2
Payments for financing element derivatives	(40.9)	(43.0)
Excess tax benefits from share-based payment arrangements		5.5
Purchase of subsidiary shares from noncontrolling interest		(2.3)
Dividends to common stockholders	(262.7)	(223.2)
Issuance of long-term debt		3.7
Net proceeds from (repayments of) short-term borrowings	(9.8)	(156.9)
Investment contract deposits	6,949.5	6,877.7
Investment contract withdrawals	(6,621.4)	(5,071.0)
Net increase (decrease) in banking operation deposits	(0.6)	47.1
Other	3.7	(0.3)
Net cash provided by (used in) financing activities	(38.0)	1,239.0
Net increase (decrease) in cash and cash equivalents	(713.2)	(309.4)
Cash and cash equivalents at beginning of period	2,719.6	2,564.8
Cash and cash equivalents at end of period	2,006.4	2,255.4
Principal Financial Group, Inc. Parent Only
Operating activities
Net cash provided by (used in) operating activities	(33.0)	(54.6)
Investing activities
Dividends and returns of capital received from (contributed to) unconsolidated entities	249.9	594.8
Net change in other investments	(2.4)	(5.1)
Net cash provided by (used in) investing activities	247.5	589.7
Financing activities
Issuance of common stock	113.3	11.2
Acquisition of treasury stock	(169.2)	(209.7)
Excess tax benefits from share-based payment arrangements		0.2
Dividends to common stockholders	(262.7)	(223.2)
Net cash provided by (used in) financing activities	(318.6)	(421.5)
Net increase (decrease) in cash and cash equivalents	(104.1)	113.6
Cash and cash equivalents at beginning of period	882.6	578.7
Cash and cash equivalents at end of period	778.5	692.3
Principal Life Insurance Company Only
Operating activities
Net cash provided by (used in) operating activities	3,345.2	1,872.7
Investing activities
Available-for-sale securities: Purchases	(6,599.1)	(7,072.5)
Available-for-sale securities: Sales	615.7	295.3
Available-for-sale securities: Maturities	4,167.0	3,383.2
Mortgage loans acquired or originated	(1,263.0)	(1,123.0)
Mortgage loans sold or repaid	743.7	776.1
Real estate sold		3.5
Net (purchases) sales of property and equipment	(63.8)	(54.8)
Dividends and returns of capital received from (contributed to) unconsolidated entities	(15.0)	1.5
Net change in other investments	14.2	183.4
Net cash provided by (used in) investing activities	(2,400.3)	(3,607.3)
Financing activities
Proceeds from financing element derivatives	0.1	0.2
Payments for financing element derivatives	(40.9)	(43.0)
Excess tax benefits from share-based payment arrangements		2.0
Capital received from (dividends and capital paid to) parent	(1,243.3)	(544.8)
Investment contract deposits	6,773.5	6,735.4
Investment contract withdrawals	(6,606.6)	(5,060.6)
Other	3.7	(0.5)
Net cash provided by (used in) financing activities	(1,113.5)	1,088.7
Net increase (decrease) in cash and cash equivalents	(168.6)	(645.9)
Cash and cash equivalents at beginning of period	675.1	1,127.9
Cash and cash equivalents at end of period	506.5	482.0
Principal Financial Services, Inc. and Other Subsidiaries Combined
Operating activities
Net cash provided by (used in) operating activities	(1,621.3)	882.8
Investing activities
Available-for-sale securities: Purchases	(637.8)	(409.7)
Available-for-sale securities: Sales	253.3	262.1
Available-for-sale securities: Maturities	454.5	284.7
Mortgage loans acquired or originated	(168.5)	(126.8)
Mortgage loans sold or repaid	127.6	137.6
Real estate acquired	(96.8)	(66.4)
Real estate sold	47.1	23.8
Net (purchases) sales of property and equipment	(40.3)	(19.9)
Dividends and returns of capital received from (contributed to) unconsolidated entities	1,243.3	544.8
Net change in other investments	(198.6)	(680.1)
Net cash provided by (used in) investing activities	983.8	(49.9)
Financing activities
Excess tax benefits from share-based payment arrangements		3.3
Purchase of subsidiary shares from noncontrolling interest	(1.3)	(2.3)
Sale of subsidiary shares to noncontrolling interest	1.3
Issuance of long-term debt	61.6	51.4
Principal repayments of long-term debt	(28.2)	(3.2)
Net proceeds from (repayments of) short-term borrowings	(9.8)	(156.9)
Capital received from (dividends and capital paid to) parent	(234.9)	(596.3)
Investment contract deposits	176.0	142.3
Investment contract withdrawals	(14.8)	(10.4)
Net increase (decrease) in banking operation deposits	(0.6)	47.1
Other		0.2
Net cash provided by (used in) financing activities	(50.7)	(524.8)
Net increase (decrease) in cash and cash equivalents	(688.2)	308.1
Cash and cash equivalents at beginning of period	2,082.8	1,253.7
Cash and cash equivalents at end of period	1,394.6	1,561.8
Eliminations, Notes Guarantor
Operating activities
Net cash provided by (used in) operating activities	138.2	(575.3)
Investing activities
Available-for-sale securities: Purchases		5.3
Available-for-sale securities: Sales	(15.7)
Mortgage loans acquired or originated	66.2	51.8
Mortgage loans sold or repaid	(28.2)	(3.3)
Dividends and returns of capital received from (contributed to) unconsolidated entities	(1,478.2)	(1,141.1)
Net change in other investments	120.6	480.8
Net cash provided by (used in) investing activities	(1,335.3)	(606.5)
Financing activities
Purchase of subsidiary shares from noncontrolling interest	1.3
Sale of subsidiary shares to noncontrolling interest	(1.3)
Issuance of long-term debt	(61.6)	(47.7)
Principal repayments of long-term debt	28.2	3.2
Capital received from (dividends and capital paid to) parent	1,478.2	1,141.1
Net cash provided by (used in) financing activities	1,444.8	1,096.6
Net increase (decrease) in cash and cash equivalents	247.7	(85.2)
Cash and cash equivalents at beginning of period	(920.9)	(395.5)
Cash and cash equivalents at end of period	$ (673.2)	$ (480.7)