Condensed Consolidating Financial Information - Guarantor, Statements of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Operating activities
Net cash provided by (used in) operating activities	$ 895.5	$ 630.9
Investing activities
Available-for-sale securities: Purchases	(3,558.6)	(2,220.1)
Available-for-sale securities: Sales	205.5	567.5
Available-for-sale securities: Maturities	1,616.0	1,694.3
Mortgage loans acquired or originated	(475.9)	(511.3)
Mortgage loans sold or repaid	479.3	408.9
Real estate acquired	(44.5)	(87.9)
Net (purchases) sales of property and equipment	(44.2)	(32.8)
Net change in other investments	(30.9)	(48.1)
Net cash provided by (used in) investing activities	(1,853.3)	(229.5)
Financing activities
Issuance of common stock	10.0	17.9
Acquisition of treasury stock	(105.6)	(75.4)
Proceeds from financing element derivatives		0.2
Payments for financing element derivatives	(21.3)	(19.3)
Excess tax benefits from share-based payment arrangements	5.0	13.0
Purchase of subsidiary shares from noncontrolling interest	(2.3)	(8.7)
Dividends to common stockholders	(110.4)	(106.3)
Dividends to preferred stockholders		(8.2)
Issuance of long-term debt	3.2	3.1
Principal repayments of long-term debt		(19.8)
Net proceeds from (repayments of) short-term borrowings	(59.9)	(0.1)
Investment contract deposits	3,976.3	1,277.8
Investment contract withdrawals	(3,166.3)	(1,871.9)
Net increase (decrease) in banking operation deposits	4.0	(13.5)
Other	3.2	(2.1)
Net cash provided by (used in) financing activities	535.9	(813.3)
Net increase (decrease) in cash and cash equivalents	(421.9)	(411.9)
Cash and cash equivalents at beginning of period	2,564.8	1,863.9
Cash and cash equivalents at end of period	2,142.9	1,452.0
Principal Financial Group, Inc. Parent Only
Operating activities
Net cash provided by (used in) operating activities	(3.4)	11.7
Investing activities
Dividends and returns of capital received from (contributions to) unconsolidated entities	369.8	225.4
Net change in other investments	(4.9)	(2.5)
Net cash provided by (used in) investing activities	364.9	222.9
Financing activities
Issuance of common stock	10.0	17.9
Acquisition of treasury stock	(105.6)	(75.4)
Excess tax benefits from share-based payment arrangements	0.1	0.4
Dividends to common stockholders	(110.4)	(106.3)
Dividends to preferred stockholders		(8.2)
Net cash provided by (used in) financing activities	(205.9)	(171.6)
Net increase (decrease) in cash and cash equivalents	155.6	63.0
Cash and cash equivalents at beginning of period	578.7	412.4
Cash and cash equivalents at end of period	734.3	475.4
Principal Life Insurance Company Only
Operating activities
Net cash provided by (used in) operating activities	998.3	594.0
Investing activities
Available-for-sale securities: Purchases	(3,374.1)	(1,976.7)
Available-for-sale securities: Sales	134.8	463.4
Available-for-sale securities: Maturities	1,486.4	1,499.5
Mortgage loans acquired or originated	(430.2)	(526.9)
Mortgage loans sold or repaid	420.4	374.4
Real estate acquired	(0.1)	(0.3)
Net (purchases) sales of property and equipment	(33.3)	(26.7)
Dividends and returns of capital received from (contributions to) unconsolidated entities	0.8	(2.8)
Net change in other investments	(43.9)	(79.8)
Net cash provided by (used in) investing activities	(1,839.2)	(275.9)
Financing activities
Proceeds from financing element derivatives		0.2
Payments for financing element derivatives	(21.3)	(19.3)
Excess tax benefits from share-based payment arrangements	1.8	4.7
Capital received from (dividends and capital paid to) parent	(319.8)	(125.4)
Investment contract deposits	3,903.7	1,204.7
Investment contract withdrawals	(3,161.9)	(1,869.7)
Other	3.2	(2.1)
Net cash provided by (used in) financing activities	405.7	(806.9)
Net increase (decrease) in cash and cash equivalents	(435.2)	(488.8)
Cash and cash equivalents at beginning of period	1,127.9	602.7
Cash and cash equivalents at end of period	692.7	113.9
Principal Financial Services, Inc. and Other Subsidiaries Combined
Operating activities
Net cash provided by (used in) operating activities	212.4	(141.8)
Investing activities
Available-for-sale securities: Purchases	(186.7)	(243.4)
Available-for-sale securities: Sales	70.7	109.9
Available-for-sale securities: Maturities	129.6	194.8
Mortgage loans acquired or originated	(56.0)	(63.6)
Mortgage loans sold or repaid	60.5	61.5
Real estate acquired	(44.4)	(87.6)
Net (purchases) sales of property and equipment	(10.9)	(6.1)
Dividends and returns of capital received from (contributions to) unconsolidated entities	319.8	125.4
Net change in other investments	(273.4)	(141.5)
Net cash provided by (used in) investing activities	9.2	(50.6)
Financing activities
Excess tax benefits from share-based payment arrangements	3.1	7.9
Purchase of subsidiary shares from noncontrolling interest	(2.3)	(8.7)
Issuance of long-term debt	11.8	70.3
Principal repayments of long-term debt	(1.6)	(47.8)
Net proceeds from (repayments of) short-term borrowings	(59.9)	(0.1)
Capital received from (dividends and capital paid to) parent	(370.6)	(222.6)
Investment contract deposits	72.6	73.1
Investment contract withdrawals	(4.4)	(2.2)
Net increase (decrease) in banking operation deposits	4.0	(13.5)
Net cash provided by (used in) financing activities	(347.3)	(143.6)
Net increase (decrease) in cash and cash equivalents	(125.7)	(336.0)
Cash and cash equivalents at beginning of period	1,253.7	1,253.6
Cash and cash equivalents at end of period	1,128.0	917.6
Eliminations, Notes Guarantor
Operating activities
Net cash provided by (used in) operating activities	(311.8)	167.0
Investing activities
Available-for-sale securities: Purchases	2.2
Available-for-sale securities: Sales		(5.8)
Mortgage loans acquired or originated	10.3	79.2
Mortgage loans sold or repaid	(1.6)	(27.0)
Dividends and returns of capital received from (contributions to) unconsolidated entities	(690.4)	(348.0)
Net change in other investments	291.3	175.7
Net cash provided by (used in) investing activities	(388.2)	(125.9)
Financing activities
Issuance of long-term debt	(8.6)	(67.2)
Principal repayments of long-term debt	1.6	28.0
Capital received from (dividends and capital paid to) parent	690.4	348.0
Net cash provided by (used in) financing activities	683.4	308.8
Net increase (decrease) in cash and cash equivalents	(16.6)	349.9
Cash and cash equivalents at beginning of period	(395.5)	(404.8)
Cash and cash equivalents at end of period	$ (412.1)	$ (54.9)