Consolidated Statements of Operations (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Revenues
Premiums and other considerations									$ 2,891.0	$ 3,555.5	$ 3,750.6
Fees and other revenues									2,565.1	2,298.1	2,096.0
Net investment income (loss)									3,375.8	3,496.5	3,400.8
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities									75.0	48.7	54.9
Total other-than-temporary impairment losses on available-for-sale securities									(147.6)	(296.3)	(714.1)
Other-than-temporary impairment losses on fixed maturities, available-for-sale reclassified to (from) other comprehensive income									(49.7)	56.1	260.9
Net impairment losses on available-for-sale securities									(197.3)	(240.2)	(453.2)
Net realized capital gains (losses)									(122.3)	(191.5)	(398.3)
Total revenues	2,059.0	2,088.8	2,342.0	2,219.8	2,372.5	2,288.5	2,233.6	2,264.0	8,709.6	9,158.6	8,849.1
Expenses
Benefits, claims and settlement expenses									4,454.1	5,338.4	5,334.5
Dividends to policyholders									210.2	219.9	242.2
Operating expenses									3,057.7	2,759.0	2,526.6
Total expenses	1,865.4	1,950.6	1,969.7	1,936.3	2,108.8	2,125.0	2,075.8	2,007.7	7,722.0	8,317.3	8,103.3
Income (loss) before income taxes									987.6	841.3	745.8
Income taxes (benefits)									236.4	124.1	100.1
Net income (loss)	171.9	66.3	289.9	223.1	218.1	151.3	144.2	203.6	751.2	717.2	645.7
Net income (loss) attributable to noncontrolling interest									36.2	17.9	23.0
Net income (loss) attributable to Principal Financial Group, Inc.									715.0	699.3	622.7
Preferred stock dividends									33.0	33.0	33.0
Net income (loss) available to common stockholders	$ 164.0	$ 63.7	$ 258.0	$ 196.3	$ 199.3	$ 142.2	$ 134.0	$ 190.8	$ 682.0	$ 666.3	$ 589.7
Earnings per common share
Basic earnings per common share (in dollars per share)	$ 0.54	$ 0.20	$ 0.81	$ 0.61	$ 0.62	$ 0.44	$ 0.42	$ 0.60	$ 2.17	$ 2.08	$ 1.98
Diluted earnings per common share (in dollars per share)	$ 0.54	$ 0.20	$ 0.80	$ 0.60	$ 0.62	$ 0.44	$ 0.42	$ 0.59	$ 2.15	$ 2.06	$ 1.97