Segment Information - Reconciliation of Segment Operating Revenues and Earnings to Consolidated (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Operating Revenue And Profit (Loss) From Segments To Consolidated
Net realized capital gains (losses), net of related revenue adjustments	$ 113.6	$ 45.6
Total revenues	3,036.6	2,657.3
Pre-tax net realized capital gains (losses), as adjusted	96.8	33.5
Income (loss) before income taxes	439.8	458.1
Impact of court ruling on uncertain tax positions		16.9
Pre-tax gains (losses) associated with exited group medical insurance business that does not qualify for discontinued operations		(0.3)
Retirement and Income Solutions
Operating Revenue And Profit (Loss) From Segments To Consolidated
Inter-segment revenues	89.2	105.2
Principal Global Investors
Operating Revenue And Profit (Loss) From Segments To Consolidated
Inter-segment revenues	54.3	60.4
Operating Segments
Operating Revenue And Profit (Loss) From Segments To Consolidated
Operating revenues	2,937.9	2,559.2
Pre-tax operating earnings (losses)	356.3	420.9
Operating Segments | Retirement and Income Solutions
Operating Revenue And Profit (Loss) From Segments To Consolidated
Operating revenues	1,464.0	1,124.5
Pre-tax operating earnings (losses)	181.4	202.9
Operating Segments | Retirement and Income Solutions | Retirement and Income Solutions - Fee
Operating Revenue And Profit (Loss) From Segments To Consolidated
Operating revenues	413.3	437.7
Operating Segments | Retirement and Income Solutions | Retirement and Income Solutions - Spread
Operating Revenue And Profit (Loss) From Segments To Consolidated
Operating revenues	1,050.7	686.8
Operating Segments | Principal Global Investors
Operating Revenue And Profit (Loss) From Segments To Consolidated
Operating revenues	309.5	328.9
Pre-tax operating earnings (losses)	79.7	92.6
Operating Segments | Principal International
Operating Revenue And Profit (Loss) From Segments To Consolidated
Operating revenues	286.6	250.8
Pre-tax operating earnings (losses)	68.0	80.8
Operating Segments | U.S. Insurance Solutions
Operating Revenue And Profit (Loss) From Segments To Consolidated
Operating revenues	890.9	861.7
Pre-tax operating earnings (losses)	80.5	81.9
Operating Segments | U.S. Insurance Solutions | Specialty benefits insurance
Operating Revenue And Profit (Loss) From Segments To Consolidated
Operating revenues	482.9	466.3
Operating Segments | U.S. Insurance Solutions | Individual life insurance
Operating Revenue And Profit (Loss) From Segments To Consolidated
Operating revenues	408.0	395.4
Operating Segments | Corporate
Operating Revenue And Profit (Loss) From Segments To Consolidated
Operating revenues	(13.1)	(6.7)
Pre-tax operating earnings (losses)	(53.3)	(37.3)
Reconciling Items
Operating Revenue And Profit (Loss) From Segments To Consolidated
Net realized capital gains (losses), net of related revenue adjustments	113.6	45.6
Certain revenue adjustments related to equity method investments	(14.9)	(14.6)
Other income on a tax indemnification		66.9
Exited group medical insurance business		0.2
Pre-tax net realized capital gains (losses), as adjusted	96.8	33.5
Pre-tax other adjustments		16.6
Certain earnings adjustments related to equity method investments and noncontrolling interest	$ (13.3)	$ (12.9)