Segment Information (Details 4) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Operating Revenues for Products and Services
Operating revenues	$ 2,638.6	$ 2,031.7	$ 6,882.3	$ 6,177.7
Net realized capital gains (losses), net of related revenue adjustments	64.9	(55.8)	12.8	(124.1)
Exited group medical insurance business	1.2	117.7	24.1	553.4
Assumption change within Individual Life business				4.9
Total revenues	2,704.7	2,093.6	6,919.2	6,611.9
Retirement and Investor Services
Operating Revenues for Products and Services
Operating revenues	1,570.7	996.0	3,707.0	3,058.1
Retirement and Investor Services | Total Accumulation
Operating Revenues for Products and Services
Operating revenues	793.6	747.6	2,310.4	2,272.5
Retirement and Investor Services | Full service accumulation
Operating Revenues for Products and Services
Operating revenues	339.0	332.3	1,006.1	1,018.0
Retirement and Investor Services | Principal Funds
Operating Revenues for Products and Services
Operating revenues	156.8	138.8	453.0	424.6
Retirement and Investor Services | Individual annuities
Operating Revenues for Products and Services
Operating revenues	302.1	278.2	864.0	840.6
Retirement and Investor Services | Bank and trust services
Operating Revenues for Products and Services
Operating revenues	26.1	26.0	75.9	74.4
Retirement and Investor Services | Accumulation Eliminations
Operating Revenues for Products and Services
Operating revenues	(30.4)	(27.7)	(88.6)	(85.1)
Retirement and Investor Services | Total Guaranteed
Operating Revenues for Products and Services
Operating revenues	777.1	248.4	1,396.6	785.6
Retirement and Investor Services | Investment only
Operating Revenues for Products and Services
Operating revenues	105.2	123.1	328.8	386.5
Retirement and Investor Services | Full service payout
Operating Revenues for Products and Services
Operating revenues	671.9	125.3	1,067.8	399.1
Principal Global Investors
Operating Revenues for Products and Services
Operating revenues	144.0	132.9	423.2	394.5
Inter-segment revenues	54.1	54.0	160.0	161.5
Principal International
Operating Revenues for Products and Services
Operating revenues	203.8	220.1	676.9	653.4
U.S. Insurance Solutions
Operating Revenues for Products and Services
Operating revenues	766.5	739.3	2,215.0	2,202.0
U.S. Insurance Solutions | Individual life insurance
Operating Revenues for Products and Services
Operating revenues	372.6	362.9	1,044.8	1,074.7
U.S. Insurance Solutions | Specialty benefits insurance
Operating Revenues for Products and Services
Operating revenues	393.9	376.4	1,170.2	1,127.3
Corporate
Operating Revenues for Products and Services
Operating revenues	$ (46.4)	$ (56.6)	$ (139.8)	$ (130.3)