Condensed Consolidating Financial Information (Details 3) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Operating activities
Net cash provided by (used in) operating activities	$ 2,713.3	$ 2,791.7	$ 2,243.0
Investing activities
Available-for-sale securities: Purchases	(6,742.4)	(7,187.9)	(7,933.3)
Available-for-sale securities: Sales	980.7	1,684.6	3,439.8
Available-for-sale securities: Maturities	5,760.8	5,161.3	4,568.1
Mortgage loans acquired or originated	(1,484.9)	(1,272.0)	(586.5)
Mortgage loans sold or repaid	1,793.1	1,798.0	1,704.4
Real estate acquired	(129.9)	(53.8)	(62.2)
Net (purchases) sales of property and equipment	(56.9)	(21.5)	(26.2)
Purchases of interest in subsidiaries, net of cash acquired	(270.5)		(45.7)
Net change in other investments	(52.1)	(81.2)	(31.6)
Net cash provided by (used in) investing activities	(202.1)	27.5	1,026.8
Financing activities
Issuance of common stock	25.9	20.6	1,123.0
Acquisition of treasury stock	(556.4)	(2.6)	(4.1)
Proceeds from financing element derivatives	75.9	79.3	122.0
Payments for financing element derivatives	(46.5)	(46.5)	(67.4)
Excess tax benefits from share-based payment arrangements	2.0	1.0	0.2
Dividends to common stockholders	(213.7)	(176.2)	(159.5)
Dividends to preferred stockholders	(33.0)	(33.0)	(33.0)
Issuance of long-term debt		2.3	745.1
Principal repayments of long-term debt	(12.2)	(11.1)	(468.2)
Net proceeds from (repayments of) short-term borrowings	3.2	1.7	(405.1)
Investment contract deposits	6,302.1	4,283.8	4,224.1
Investment contract withdrawals	(7,079.0)	(7,343.4)	(8,752.7)
Net increase (decrease) in banking operation deposits	(18.5)	46.2	43.9
Other	(4.5)	(4.3)	(5.7)
Net cash provided by (used in) financing activities	(1,554.7)	(3,182.2)	(3,637.4)
Net increase (decrease) in cash and cash equivalents	956.5	(363.0)	(367.6)
Cash and cash equivalents at beginning of period	1,877.4	2,240.4	2,608.0
Cash and cash equivalents at end of period	2,833.9	1,877.4	2,240.4
Principal Financial Group, Inc. Parent Only
Operating activities
Net cash provided by (used in) operating activities	(73.3)	29.3	(442.6)
Investing activities
Available-for-sale securities: Purchases	(4.4)	(286.8)	(187.6)
Available-for-sale securities: Sales	200.0	95.5	4.0
Available-for-sale securities: Maturities	4.4	117.9	57.1
Net (purchases) sales of property and equipment			(0.1)
Dividends and returns of capital received from (contributions to) unconsolidated entities	506.5	301.8	(795.9)
Net change in other investments	(0.2)	(0.2)	0.2
Net cash provided by (used in) investing activities	706.3	228.2	(922.3)
Financing activities
Issuance of common stock	25.9	20.6	1,123.0
Acquisition of treasury stock	(556.4)	(2.6)	(4.1)
Dividends to common stockholders	(213.7)	(176.2)	(159.5)
Dividends to preferred stockholders	(33.0)	(33.0)	(33.0)
Issuance of long-term debt			745.1
Net cash provided by (used in) financing activities	(777.2)	(191.2)	1,671.5
Net increase (decrease) in cash and cash equivalents	(144.2)	66.3	306.6
Cash and cash equivalents at beginning of period	370.9	304.6	(2.0)
Cash and cash equivalents at end of period	226.7	370.9	304.6
Principal Life Insurance Company Only
Operating activities
Net cash provided by (used in) operating activities	2,495.5	2,374.5	2,616.9
Investing activities
Available-for-sale securities: Purchases	(6,047.2)	(5,631.7)	(6,537.4)
Available-for-sale securities: Sales	689.5	1,240.8	3,002.0
Available-for-sale securities: Maturities	5,037.0	4,390.7	3,944.6
Mortgage loans acquired or originated	(1,372.7)	(1,209.4)	(507.3)
Mortgage loans sold or repaid	1,548.6	1,624.1	1,449.3
Real estate acquired	(0.3)	(0.2)
Net (purchases) sales of property and equipment	(46.7)	(4.3)	(11.1)
Dividends and returns of capital received from (contributions to) unconsolidated entities	327.9	229.1	(29.6)
Net change in other investments	(48.1)	(46.8)	55.8
Net cash provided by (used in) investing activities	88.0	592.3	1,366.3
Financing activities
Proceeds from financing element derivatives	75.9	79.3	122.0
Payments for financing element derivatives	(46.5)	(46.5)	(67.4)
Excess tax benefits from share-based payment arrangements	0.8	0.4	0.1
Capital received from (dividends and capital paid to) parent	(756.5)	(301.8)	150.9
Investment contract deposits	5,868.7	4,100.0	4,220.2
Investment contract withdrawals	(7,076.7)	(7,343.3)	(8,752.7)
Other	(4.5)	(4.3)	(5.7)
Net cash provided by (used in) financing activities	(1,938.8)	(3,516.2)	(4,332.6)
Net increase (decrease) in cash and cash equivalents	644.7	(549.4)	(349.4)
Cash and cash equivalents at beginning of period	699.8	1,249.2	1,598.6
Cash and cash equivalents at end of period	1,344.5	699.8	1,249.2
Principal Financial Services, Inc. and Other Subsidiaries Combined
Operating activities
Net cash provided by (used in) operating activities	374.7	414.8	(129.2)
Investing activities
Available-for-sale securities: Purchases	(696.8)	(1,273.5)	(1,288.3)
Available-for-sale securities: Sales	116.2	380.3	602.9
Available-for-sale securities: Maturities	719.4	652.7	566.4
Mortgage loans acquired or originated	(169.0)	(295.8)	(151.3)
Mortgage loans sold or repaid	339.3	422.6	366.2
Real estate acquired	(129.6)	(53.6)	(62.2)
Net (purchases) sales of property and equipment	(10.2)	(17.2)	(15.0)
Purchases of interest in subsidiaries, net of cash acquired	(270.5)		(45.7)
Dividends and returns of capital received from (contributions to) unconsolidated entities	756.5	301.8	(150.9)
Net change in other investments	14.5	(163.3)	24.7
Net cash provided by (used in) investing activities	669.8	(46.0)	(153.2)
Financing activities
Excess tax benefits from share-based payment arrangements	1.2	0.6	0.1
Issuance of long-term debt		27.6
Principal repayments of long-term debt	(69.4)	(58.7)	(665.1)
Net proceeds from (repayments of) short-term borrowings	3.2	(30.4)	(405.2)
Capital received from (dividends and capital paid to) parent	(834.4)	(530.9)	825.5
Investment contract deposits	433.4	183.8	3.9
Investment contract withdrawals	(2.3)	(0.1)
Net increase (decrease) in banking operation deposits	(18.5)	46.2	43.9
Net cash provided by (used in) financing activities	(486.8)	(361.9)	(196.9)
Net increase (decrease) in cash and cash equivalents	557.7	6.9	(479.3)
Cash and cash equivalents at beginning of period	719.9	713.0	1,192.3
Cash and cash equivalents at end of period	1,277.6	719.9	713.0
Eliminations, Notes Guarantor
Operating activities
Net cash provided by (used in) operating activities	(83.6)	(26.9)	197.9
Investing activities
Available-for-sale securities: Purchases	6.0	4.1	80.0
Available-for-sale securities: Sales	(25.0)	(32.0)	(169.1)
Mortgage loans acquired or originated	56.8	233.2	72.1
Mortgage loans sold or repaid	(94.8)	(248.7)	(111.1)
Dividends and returns of capital received from (contributions to) unconsolidated entities	(1,590.9)	(832.7)	976.4
Net change in other investments	(18.3)	129.1	(112.3)
Net cash provided by (used in) investing activities	(1,666.2)	(747.0)	736.0
Financing activities
Issuance of long-term debt		(25.3)
Principal repayments of long-term debt	57.2	47.6	196.9
Net proceeds from (repayments of) short-term borrowings		32.1	0.1
Capital received from (dividends and capital paid to) parent	1,590.9	832.7	(976.4)
Net cash provided by (used in) financing activities	1,648.1	887.1	(779.4)
Net increase (decrease) in cash and cash equivalents	(101.7)	113.2	154.5
Cash and cash equivalents at beginning of period	86.8	(26.4)	(180.9)
Cash and cash equivalents at end of period	$ (14.9)	$ 86.8	$ (26.4)