Segment Information (Details 2) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Segment Reporting Information
Net realized capital gains (losses), net of related revenue adjustments	$ (141.3)		$ 64.3	$ (95.7)	$ 43.2
Other income on a tax indemnification	(6.7)			60.2
Total revenues	3,259.2		2,606.7	5,916.5	5,085.3
Net realized capital gains (losses), as adjusted	(82.8)		30.8	(69.7)	7.9
Net income (loss) available to common stockholders	241.1		306.3	655.3	600.0
Change in deferred tax balances related to merger of legal entities				105.2
Impact of court ruling on uncertain tax positions		$ (30.3)	(47.5)	(30.3)	(47.5)
After-tax gains (losses) associated with exited group medical insurance business that does not qualify for discontinued operations			(0.1)	(0.2)	(0.6)
Operating Segments
Segment Reporting Information
Operating revenues	3,406.8		2,542.6	5,951.4	5,041.9
Operating earnings (loss)	323.9		323.1	650.3	640.2
Operating Segments | Retirement and Investor Services
Segment Reporting Information
Operating revenues	2,108.6		1,265.1	3,428.0	2,526.3
Operating earnings (loss)	216.7		212.1	427.6	428.6
Operating Segments | Principal Global Investors
Segment Reporting Information
Operating revenues	188.4		173.2	375.0	344.3
Operating earnings (loss)	31.5		27.4	62.2	54.3
Operating Segments | Principal International
Segment Reporting Information
Operating revenues	315.0		357.3	551.2	657.7
Operating earnings (loss)	59.3		68.0	119.2	131.3
Operating Segments | U.S. Insurance Solutions
Segment Reporting Information
Operating revenues	857.8		811.4	1,719.5	1,615.8
Operating earnings (loss)	58.8		49.0	113.9	92.4
Operating Segments | Corporate
Segment Reporting Information
Operating revenues	(63.0)		(64.4)	(122.3)	(102.2)
Operating earnings (loss)	(42.4)		(33.4)	(72.6)	(66.4)
Reconciling Items
Segment Reporting Information
Net realized capital gains (losses), net of related revenue adjustments	(141.3)		64.3	(95.7)	43.2
Other income on a tax indemnification	(6.7)			60.2
Exited group medical insurance business	0.4		(0.2)	0.6	0.2
Net realized capital gains (losses), as adjusted	$ (82.8)		30.8	(69.7)	7.9
Other after-tax adjustments			$ (47.6)	$ 74.7	$ (48.1)