Stockholders' Equity (Details 3) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Change in accumulated other comprehensive income (loss) rollforward
Accumulated other comprehensive income (loss), balance at end of period	$ 488.1		$ 258.0
Net unrealized gains (losses) on available-for-sale securities
Change in accumulated other comprehensive income (loss) rollforward
Accumulated other comprehensive income (loss), balance at beginning of period	860.7	652.1	652.1
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.	161.3	175.4	175.4
Accumulated other comprehensive income (loss), balance at end of period	1,022.0	827.5	860.7
Noncredit component of impairment losses on fixed maturities available-for-sale
Change in accumulated other comprehensive income (loss) rollforward
Accumulated other comprehensive income (loss), balance at beginning of period	(167.2)	(198.2)	(198.2)
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.	(0.9)	17.3
Accumulated other comprehensive income (loss), balance at end of period	(168.1)	(180.9)
Net unrealized gains (losses) on derivative instruments
Change in accumulated other comprehensive income (loss) rollforward
Accumulated other comprehensive income (loss), balance at beginning of period	34.9	11.3	11.3
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.	(3.5)	(4.4)
Accumulated other comprehensive income (loss), balance at end of period	31.4	6.9
Foreign currency translation adjustment
Change in accumulated other comprehensive income (loss) rollforward
Accumulated other comprehensive income (loss), balance at beginning of period	(109.3)	29.7	29.7
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.	64.5	21.8
Accumulated other comprehensive income (loss), balance at end of period	(44.8)	51.5
Unrecognized postretirement benefit obligations
Change in accumulated other comprehensive income (loss) rollforward
Accumulated other comprehensive income (loss), balance at beginning of period	(361.1)	(188.2)	(188.2)
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.	8.7	49.2
Accumulated other comprehensive income (loss), balance at end of period	(352.4)	(139.0)
Accumulated other comprehensive income (loss)
Change in accumulated other comprehensive income (loss) rollforward
Accumulated other comprehensive income (loss), balance at beginning of period	258.0	306.7	306.7
Other comprehensive income (loss) attributable to Principal Financial Group, Inc.	230.1	259.3
Accumulated other comprehensive income (loss), balance at end of period	$ 488.1	$ 566.0