Condensed Consolidating Financial Information (Details 6) - 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 Financial Services, Inc. Only
Operating activities
Net cash provided by (used in) operating activities	185.2	(373.5)
Investing activities
Dividends and returns of capital received from (contributions to) unconsolidated entities	212.7	714.0
Net change in other investments	(458.4)	(98.7)
Net cash provided by (used in) investing activities	(245.7)	615.3
Financing activities
Issuance of long-term debt	156.0
Capital received from (dividends and capital paid to) parent	(425.5)	(517.5)
Net cash provided by (used in) financing activities	(269.5)	(517.5)
Net increase (decrease) in cash and cash equivalents	(330.0)	(275.7)
Cash and cash equivalents at beginning of period	907.2	688.7
Cash and cash equivalents at end of period	577.2	413.0
Principal Life Insurance Company and Other Subsidiaries Combined
Operating activities
Net cash provided by (used in) operating activities	3,462.8	2,335.4
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	45.5	185.2
Net cash provided by (used in) investing activities	(1,704.3)	(698.7)
Financing activities
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	14.5	8.3
Purchase of subsidiary shares from noncontrolling interest	(22.5)	(222.4)
Issuance of long-term debt	50.9	36.5
Principal repayments of long-term debt	(94.9)	(100.3)
Net proceeds from (repayments of) short-term borrowings	188.4	(113.5)
Capital received from (dividends and capital paid to) parent	(212.7)	(714.0)
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	(906.5)	(2,824.3)
Net increase (decrease) in cash and cash equivalents	852.0	(1,187.6)
Cash and cash equivalents at beginning of period	1,598.0	2,384.0
Cash and cash equivalents at end of period	2,450.0	1,196.4
Eliminations, Shelf Registration Debt Guarantor
Operating activities
Net cash provided by (used in) operating activities	(284.6)	278.0
Investing activities
Dividends and returns of capital received from (contributions to) unconsolidated entities	(638.2)	(1,231.5)
Net change in other investments	394.4	(27.9)
Net cash provided by (used in) investing activities	(243.8)	(1,259.4)
Financing activities
Issuance of long-term debt	(198.3)
Principal repayments of long-term debt	42.3
Net proceeds from (repayments of) short-term borrowings	8.8	93.0
Capital received from (dividends and capital paid to) parent	638.2	1,231.5
Net cash provided by (used in) financing activities	491.0	1,324.5
Net increase (decrease) in cash and cash equivalents	(37.4)	343.1
Cash and cash equivalents at beginning of period	(1,053.7)	(832.4)
Cash and cash equivalents at end of period	$ (1,091.1)	$ (489.3)