Condensed Consolidating Financial Information (Details 3) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Operating activities
Net cash provided by (used in) operating activities	$ 3,323.0	$ 2,197.6
Investing activities
Available-for-sale securities: Purchases	(7,052.6)	(6,596.0)
Available-for-sale securities: Sales	1,410.8	1,871.5
Available-for-sale securities: Maturities	5,020.5	4,499.3
Mortgage loans acquired or originated	(1,658.0)	(1,614.2)
Mortgage loans sold or repaid	1,175.3	1,319.8
Real estate acquired	(255.8)	(246.8)
Net (purchases) sales of property and equipment	(96.3)	(117.5)
Purchases of interest in subsidiaries, net of cash acquired	(293.7)
Net change in other investments	(17.0)	56.6
Net cash provided by (used in) investing activities	(1,766.8)	(827.3)
Financing activities
Issuance of common stock	71.5	69.6
Acquisition of treasury stock	(188.2)	(222.5)
Proceeds from financing element derivatives	0.2	14.9
Payments for financing element derivatives	(60.9)	(41.5)
Excess tax benefits from share-based payment arrangements	15.2	8.6
Purchase of subsidiary shares from noncontrolling interest	(22.5)	(222.4)
Dividends to common stockholders	(330.2)	(276.7)
Dividends to preferred stockholders	(16.5)	(24.7)
Preferred stock redemption	(550.0)
Issuance of long-term debt	800.4	36.5
Principal repayments of long-term debt	(52.6)	(100.3)
Net proceeds from (repayments of) short-term borrowings	197.2	(20.5)
Investment contract deposits	4,190.3	4,184.3
Investment contract withdrawals	(4,974.3)	(5,864.3)
Net increase (decrease) in banking operation deposits	24.9	(2.3)
Other	(10.4)	(10.0)
Net cash provided by (used in) financing activities	(905.9)	(2,471.3)
Net increase (decrease) in cash and cash equivalents	650.3	(1,101.0)
Cash and cash equivalents at beginning of period	1,863.9	2,371.8
Cash and cash equivalents at end of period	2,514.2	1,270.8
Principal Financial Group, Inc. Parent Only
Operating activities
Net cash provided by (used in) operating activities	(40.4)	(42.3)
Investing activities
Dividends and returns of capital received from (contributions to) unconsolidated entities	425.5	517.5
Net change in other investments	1.5	(2.0)
Net cash provided by (used in) investing activities	427.0	515.5
Financing activities
Issuance of common stock	71.5	69.6
Acquisition of treasury stock	(188.2)	(222.5)
Excess tax benefits from share-based payment arrangements	0.7	0.3
Dividends to common stockholders	(330.2)	(276.7)
Dividends to preferred stockholders	(16.5)	(24.7)
Preferred stock redemption	(550.0)
Issuance of long-term debt	791.8
Net cash provided by (used in) financing activities	(220.9)	(454.0)
Net increase (decrease) in cash and cash equivalents	165.7	19.2
Cash and cash equivalents at beginning of period	412.4	131.5
Cash and cash equivalents at end of period	578.1	150.7
Principal Life Insurance Company Only
Operating activities
Net cash provided by (used in) operating activities	3,038.3	1,729.9
Investing activities
Available-for-sale securities: Purchases	(6,241.4)	(5,902.2)
Available-for-sale securities: Sales	834.3	1,559.7
Available-for-sale securities: Maturities	4,436.6	4,149.4
Mortgage loans acquired or originated	(1,573.8)	(1,620.6)
Mortgage loans sold or repaid	1,011.2	1,291.5
Real estate acquired	(0.3)	(0.6)
Net (purchases) sales of property and equipment	(77.7)	(90.3)
Dividends and returns of capital received from (contributions to) unconsolidated entities	(1.5)	263.5
Net change in other investments	169.7	(83.6)
Net cash provided by (used in) investing activities	(1,442.9)	(433.2)
Financing activities
Issuance of common stock		(0.2)
Proceeds from financing element derivatives	0.2	14.9
Payments for financing element derivatives	(60.9)	(41.5)
Excess tax benefits from share-based payment arrangements	5.5	3.7
Principal repayments of long-term debt		(100.0)
Capital received from (dividends and capital paid to) parent	(225.6)	(467.5)
Investment contract deposits	3,988.0	3,978.2
Investment contract withdrawals	(4,966.3)	(5,857.3)
Other	(10.4)	(10.1)
Net cash provided by (used in) financing activities	(1,269.5)	(2,479.8)
Net increase (decrease) in cash and cash equivalents	325.9	(1,183.1)
Cash and cash equivalents at beginning of period	602.7	1,332.2
Cash and cash equivalents at end of period	928.6	149.1
Principal Financial Services, Inc. and Other Subsidiaries Combined
Operating activities
Net cash provided by (used in) operating activities	316.7	504.6
Investing activities
Available-for-sale securities: Purchases	(811.3)	(705.1)
Available-for-sale securities: Sales	576.5	311.8
Available-for-sale securities: Maturities	583.9	349.9
Mortgage loans acquired or originated	(246.5)	(118.1)
Mortgage loans sold or repaid	205.1	137.6
Real estate acquired	(255.5)	(246.2)
Net (purchases) sales of property and equipment	(18.6)	(27.2)
Purchases of interest in subsidiaries, net of cash acquired	(293.7)
Dividends and returns of capital received from (contributions to) unconsolidated entities	225.6	467.5
Net change in other investments	(167.1)	34.9
Net cash provided by (used in) investing activities	(201.6)	205.1
Financing activities
Issuance of common stock		0.2
Excess tax benefits from share-based payment arrangements	9.0	4.6
Purchase of subsidiary shares from noncontrolling interest	(23.1)	(222.7)
Sale of subsidiary shares to noncontrolling interest	0.6	0.3
Issuance of long-term debt	167.2	138.9
Principal repayments of long-term debt	(94.8)	(87.4)
Net proceeds from (repayments of) short-term borrowings	197.2	(20.5)
Capital received from (dividends and capital paid to) parent	(424.0)	(781.0)
Investment contract deposits	202.3	206.1
Investment contract withdrawals	(8.0)	(7.0)
Net increase (decrease) in banking operation deposits	24.9	(2.3)
Other		0.1
Net cash provided by (used in) financing activities	51.3	(770.7)
Net increase (decrease) in cash and cash equivalents	166.4	(61.0)
Cash and cash equivalents at beginning of period	1,253.6	894.5
Cash and cash equivalents at end of period	1,420.0	833.5
Eliminations, Notes Guarantor
Operating activities
Net cash provided by (used in) operating activities	8.4	5.4
Investing activities
Available-for-sale securities: Purchases	0.1	11.3
Mortgage loans acquired or originated	162.3	124.5
Mortgage loans sold or repaid	(41.0)	(109.3)
Dividends and returns of capital received from (contributions to) unconsolidated entities	(649.6)	(1,248.5)
Net change in other investments	(21.1)	107.3
Net cash provided by (used in) investing activities	(549.3)	(1,114.7)
Financing activities
Purchase of subsidiary shares from noncontrolling interest	0.6	0.3
Sale of subsidiary shares to noncontrolling interest	(0.6)	(0.3)
Issuance of long-term debt	(158.6)	(102.4)
Principal repayments of long-term debt	42.2	87.1
Capital received from (dividends and capital paid to) parent	649.6	1,248.5
Net cash provided by (used in) financing activities	533.2	1,233.2
Net increase (decrease) in cash and cash equivalents	(7.7)	123.9
Cash and cash equivalents at beginning of period	(404.8)	13.6
Cash and cash equivalents at end of period	$ (412.5)	$ 137.5