Condensed Consolidating Financial Information (Details 3) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011
Operating activities
Net cash provided by (used in) operating activities	$ 2,373.7	$ 1,885.4
Investing activities
Available-for-sale securities: Purchases	(6,467.5)	(5,179.4)
Available-for-sale securities: Sales	946.5	845.6
Available-for-sale securities: Maturities	4,702.7	4,350.3
Mortgage loans acquired or originated	(1,811.8)	(1,070.4)
Mortgage loans sold or repaid	1,166.7	1,269.6
Real estate acquired	(114.8)	(43.8)
Net (purchases) sales of property and equipment	(35.6)	(32.6)
Purchases of interest in subsidiaries, net of cash acquired	(62.5)
Net change in other investments	(42.9)	(228.4)
Net cash provided by (used in) investing activities	(1,719.2)	(89.1)
Financing activities
Issuance of common stock	24.7	23.2
Acquisition of treasury stock	(272.7)	(456.4)
Proceeds from financing element derivatives	51.6	74.7
Payments for financing element derivatives	(38.3)	(38.6)
Excess tax benefits from share-based payment arrangements	9.8	2.0
Dividends to common stockholders	(169.6)
Dividends to preferred stockholders	(16.5)	(24.7)
Issuance of long-term debt	602.9	2.6
Principal repayments of long-term debt	(2.1)	(4.3)
Net proceeds from (repayments of) short-term borrowings	(81.0)	(30.7)
Investment contract deposits	4,798.7	4,067.3
Investment contract withdrawals	(6,126.0)	(5,836.3)
Net increase (decrease) in banking operation deposits	14.9	(15.2)
Other	(5.8)	(3.1)
Net cash provided by (used in) financing activities	(1,209.4)	(2,239.5)
Net increase (decrease) in cash and cash equivalents	(554.9)	(443.2)
Cash and cash equivalents at beginning of period	2,833.9	1,877.4
Cash and cash equivalents at end of period	2,279.0	1,434.2
Principal Financial Group, Inc. Parent Only
Operating activities
Net cash provided by (used in) operating activities	49.2	(249.2)
Investing activities
Available-for-sale securities: Purchases		(4.4)
Available-for-sale securities: Sales		200.0
Available-for-sale securities: Maturities		4.4
Dividends and returns of capital received from (contributions to) unconsolidated entities	539.8	506.3
Net change in other investments	(0.2)	(0.2)
Net cash provided by (used in) investing activities	539.6	706.1
Financing activities
Issuance of common stock	24.7	23.2
Acquisition of treasury stock	(272.7)	(456.4)
Dividends to common stockholders	(169.6)
Dividends to preferred stockholders	(16.5)	(24.7)
Issuance of long-term debt	593.8
Net cash provided by (used in) financing activities	159.7	(457.9)
Net increase (decrease) in cash and cash equivalents	748.5	(1.0)
Cash and cash equivalents at beginning of period	226.7	370.9
Cash and cash equivalents at end of period	975.2	369.9
Principal Life Insurance Company Only
Operating activities
Net cash provided by (used in) operating activities	2,369.3	1,983.6
Investing activities
Available-for-sale securities: Purchases	(5,753.7)	(4,697.5)
Available-for-sale securities: Sales	862.7	545.0
Available-for-sale securities: Maturities	4,058.9	3,825.7
Mortgage loans acquired or originated	(1,687.2)	(989.0)
Mortgage loans sold or repaid	1,104.6	1,106.6
Net (purchases) sales of property and equipment	(22.4)	(27.0)
Dividends and returns of capital received from (contributions to) unconsolidated entities	435.0	224.4
Net change in other investments	(14.8)	(51.4)
Net cash provided by (used in) investing activities	(1,016.9)	(63.2)
Financing activities
Proceeds from financing element derivatives	51.6	74.7
Payments for financing element derivatives	(38.3)	(38.6)
Excess tax benefits from share-based payment arrangements	4.7	0.8
Capital received from (dividends and capital paid to) parent	(539.8)	(506.4)
Investment contract deposits	4,481.7	3,746.8
Investment contract withdrawals	(6,123.8)	(5,834.3)
Other	(5.8)	(3.1)
Net cash provided by (used in) financing activities	(2,169.7)	(2,560.1)
Net increase (decrease) in cash and cash equivalents	(817.3)	(639.7)
Cash and cash equivalents at beginning of period	1,344.5	699.8
Cash and cash equivalents at end of period	527.2	60.1
Principal Financial Services, Inc. and Other Subsidiaries Combined
Operating activities
Net cash provided by (used in) operating activities	(204.2)	447.8
Investing activities
Available-for-sale securities: Purchases	(739.5)	(480.4)
Available-for-sale securities: Sales	98.8	110.1
Available-for-sale securities: Maturities	643.8	520.2
Mortgage loans acquired or originated	(177.4)	(114.2)
Mortgage loans sold or repaid	280.3	242.1
Real estate acquired	(114.8)	(43.8)
Net (purchases) sales of property and equipment	(13.2)	(5.6)
Purchases of interest in subsidiaries, net of cash acquired	(62.5)
Dividends and returns of capital received from (contributions to) unconsolidated entities	539.8	506.4
Net change in other investments	(14.8)	(164.1)
Net cash provided by (used in) investing activities	440.5	570.7
Financing activities
Excess tax benefits from share-based payment arrangements	5.1	1.2
Issuance of long-term debt	9.1	2.6
Principal repayments of long-term debt	(45.7)	(55.8)
Net proceeds from (repayments of) short-term borrowings	(81.0)	(30.7)
Capital received from (dividends and capital paid to) parent	(974.8)	(730.7)
Investment contract deposits	317.0	320.5
Investment contract withdrawals	(2.2)	(2.0)
Net increase (decrease) in banking operation deposits	14.9	(15.2)
Net cash provided by (used in) financing activities	(757.6)	(510.1)
Net increase (decrease) in cash and cash equivalents	(521.3)	508.4
Cash and cash equivalents at beginning of period	1,277.6	719.9
Cash and cash equivalents at end of period	756.3	1,228.3
Eliminations, Notes Guarantor
Operating activities
Net cash provided by (used in) operating activities	159.4	(296.8)
Investing activities
Available-for-sale securities: Purchases	25.7	2.9
Available-for-sale securities: Sales	(15.0)	(9.5)
Mortgage loans acquired or originated	52.8	32.8
Mortgage loans sold or repaid	(218.2)	(79.1)
Dividends and returns of capital received from (contributions to) unconsolidated entities	(1,514.6)	(1,237.1)
Net change in other investments	(13.1)	(12.7)
Net cash provided by (used in) investing activities	(1,682.4)	(1,302.7)
Financing activities
Principal repayments of long-term debt	43.6	51.5
Capital received from (dividends and capital paid to) parent	1,514.6	1,237.1
Net cash provided by (used in) financing activities	1,558.2	1,288.6
Net increase (decrease) in cash and cash equivalents	35.2	(310.9)
Cash and cash equivalents at beginning of period	(14.9)	86.8
Cash and cash equivalents at end of period	$ 20.3	$ (224.1)