Schedule II - Condensed Financial Information of Registrant (Parent Only) (Details 3) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Operating activities
Net income (loss)	$ 715.0	$ 699.3	$ 622.7
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Net realized capital (gains) losses	122.3	191.5	398.3
Net cash flows for trading securities	110.8	188.3	(127.4)
Stock-based compensation	43.4	47.6	37.2
Other	525.7	189.3	12.8
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
Net (purchases) sales of property and equipment	(56.9)	(21.5)	(26.2)
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)
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
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 income (loss)	715.0	699.3	622.7
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Equity in the net (income) loss of subsidiaries	(792.4)	(750.7)	(675.1)
Net realized capital (gains) losses		(0.7)	(0.1)
Net cash flows for trading securities	21.7	111.1	(349.1)
Current and deferred income taxes (benefits)	(3.7)	(4.5)	(6.9)
Stock-based compensation	1.6	0.8	1.0
Other	(15.5)	(26.0)	(35.1)
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)
Net change in other investments	(0.2)	(0.2)	0.2
Dividends and returns of capital received from (contributions to) unconsolidated entities	506.5	301.8	(795.9)
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