Segment Information (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Segment Reporting Information
Net realized capital gains (losses), net of related revenue adjustments	$ (21.1)	$ (75.2)
Total revenues	2,478.6	2,166.7
Net realized capital gains (losses), as adjusted	(22.9)	(56.4)
After-tax gains (losses) associated with exited group medical insurance business that does not qualify for discontinued operations	(0.5)	1.4
Net income (loss) available to common stockholders	293.7	178.3
Operating Segments
Segment Reporting Information
Operating revenues	2,499.3	2,238.3
Operating earnings (loss)	317.1	233.3
Operating Segments | Retirement and Investor Services
Segment Reporting Information
Operating revenues	1,261.2	1,102.3
Operating earnings (loss)	216.5	170.0
Operating Segments | Principal Global Investors
Segment Reporting Information
Operating revenues	171.1	153.7
Operating earnings (loss)	26.9	20.3
Operating Segments | Principal International
Segment Reporting Information
Operating revenues	300.4	247.5
Operating earnings (loss)	63.3	44.6
Operating Segments | U.S. Insurance Solutions
Segment Reporting Information
Operating revenues	804.4	778.0
Operating earnings (loss)	43.4	35.7
Operating Segments | Corporate
Segment Reporting Information
Operating revenues	(37.8)	(43.2)
Operating earnings (loss)	(33.0)	(37.3)
Reconciling Items
Segment Reporting Information
Net realized capital gains (losses), net of related revenue adjustments	(21.1)	(75.2)
Exited group medical insurance business	0.4	3.6
Net realized capital gains (losses), as adjusted	(22.9)	(56.4)
Other after-tax adjustments	$ (0.5)	$ 1.4