Segment Information (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Segment Reporting Information
Operating revenues	$ 2,136.3	$ 2,098.5	$ 4,243.7	$ 4,146.0
Net realized capital gains (losses), net of related revenue adjustments	(21.7)	12.2	(52.1)	(68.3)
Exited group medical insurance business	4.0	180.8	22.9	435.7
Assumption change within Individual Life business		4.9		4.9
Total revenues	2,118.6	2,296.4	4,214.5	4,518.3
Operating earnings (loss)	216.3	229.0	429.3	448.8
Net realized capital gains (losses), as adjusted	(39.2)	23.5	(49.2)	(31.4)
Other after-tax adjustments	(4.0)	(35.2)	(5.5)	(18.1)
Net income (loss) available to common stockholders	173.1	217.3	374.6	399.3
Impact of assumption change within Individual Life business		(34.5)		(34.5)
Contribution to The Principal Financial Group Foundation, Inc.		(19.5)		(19.5)
After-tax gains (losses) associated with exited group medical insurance business that does not qualify for discontinued operations	(4.0)	18.8	(5.5)	35.9
Retirement and Investor Services
Segment Reporting Information
Operating revenues	1,081.2	1,044.2	2,136.3	2,062.1
Operating earnings (loss)	141.7	154.7	285.3	308.8
Principal Global Investors
Segment Reporting Information
Operating revenues	141.1	136.3	279.2	261.6
Operating earnings (loss)	18.2	20.8	34.4	37.4
Principal International
Segment Reporting Information
Operating revenues	210.6	227.2	473.1	433.3
Operating earnings (loss)	36.9	36.3	78.7	64.1
U.S. Insurance Solutions
Segment Reporting Information
Operating revenues	751.5	730.7	1,448.5	1,462.7
Operating earnings (loss)	50.2	49.0	100.4	102.4
Corporate
Segment Reporting Information
Operating revenues	(48.1)	(39.9)	(93.4)	(73.7)
Operating earnings (loss)	$ (30.7)	$ (31.8)	$ (69.5)	$ (63.9)