Segment Information (Details 4) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Operating Revenues for Products and Services
Total revenues	$ 2,484.3	$ 2,239.6	$ 7,569.6	$ 6,616.9
Net realized capital gains (losses), net of related revenue adjustments	(68.2)	(76.7)	(25.0)	(253.4)
Principal Global Investors
Operating Revenues for Products and Services
Inter-segment revenues	71.0	61.1	214.4	179.9
Operating Segments
Operating Revenues for Products and Services
Operating revenues	2,552.7	2,315.8	7,594.6	6,865.8
Operating Segments | Retirement and Investor Services
Operating Revenues for Products and Services
Operating revenues	1,324.7	1,136.5	3,851.0	3,387.6
Operating Segments | Retirement and Investor Services | Total Accumulation
Operating Revenues for Products and Services
Operating revenues	930.5	885.3	2,838.5	2,501.2
Operating Segments | Retirement and Investor Services | Full service accumulation
Operating Revenues for Products and Services
Operating revenues	392.1	369.9	1,170.0	1,087.7
Operating Segments | Retirement and Investor Services | Principal Funds
Operating Revenues for Products and Services
Operating revenues	219.7	194.8	638.2	565.9
Operating Segments | Retirement and Investor Services | Individual annuities
Operating Revenues for Products and Services
Operating revenues	335.6	334.5	1,082.9	881.8
Operating Segments | Retirement and Investor Services | Bank and trust services
Operating Revenues for Products and Services
Operating revenues	21.9	24.4	63.7	74.9
Operating Segments | Retirement and Investor Services | Accumulation Eliminations
Operating Revenues for Products and Services
Operating revenues	(38.8)	(38.3)	(116.3)	(109.1)
Operating Segments | Retirement and Investor Services | Total Guaranteed
Operating Revenues for Products and Services
Operating revenues	394.2	251.2	1,012.5	886.4
Operating Segments | Retirement and Investor Services | Investment only
Operating Revenues for Products and Services
Operating revenues	73.4	83.1	234.5	260.6
Operating Segments | Retirement and Investor Services | Full service payout
Operating Revenues for Products and Services
Operating revenues	320.8	168.1	778.0	625.8
Operating Segments | Principal Global Investors
Operating Revenues for Products and Services
Operating revenues	173.6	160.9	517.9	482.8
Operating Segments | Principal International
Operating Revenues for Products and Services
Operating revenues	294.5	303.3	952.2	825.9
Operating Segments | U.S. Insurance Solutions
Operating Revenues for Products and Services
Operating revenues	821.0	767.6	2,436.8	2,318.6
Operating Segments | U.S. Insurance Solutions | Individual life insurance
Operating Revenues for Products and Services
Operating revenues	382.7	362.7	1,148.5	1,110.2
Operating Segments | U.S. Insurance Solutions | Specialty benefits insurance
Operating Revenues for Products and Services
Operating revenues	438.3	405.0	1,288.4	1,208.6
Operating Segments | U.S. Insurance Solutions | U.S. Insurance Solutions Eliminations
Operating Revenues for Products and Services
Operating revenues		(0.1)	(0.1)	(0.2)
Operating Segments | Corporate
Operating Revenues for Products and Services
Operating revenues	(61.1)	(52.5)	(163.3)	(149.1)
Reconciling Items
Operating Revenues for Products and Services
Net realized capital gains (losses), net of related revenue adjustments	(68.2)	(76.7)	(25.0)	(253.4)
Exited group medical insurance business	$ (0.2)	$ 0.5		$ 4.5