Employee and Agent Benefits (Details 4) (USD $) In Millions, unless otherwise specified										12 Months Ended									12 Months Ended
	Dec. 31, 2011 Pension benefits	Dec. 31, 2010 Pension benefits	Dec. 31, 2009 Pension benefits	Dec. 31, 2011 Qualified defined benefit pension plans	Dec. 31, 2010 Qualified defined benefit pension plans	Dec. 31, 2011 Qualified defined benefit pension plans Direct real estate investments	Dec. 31, 2010 Qualified defined benefit pension plans Direct real estate investments	Dec. 31, 2011 Qualified defined benefit pension plans Fixed income security portfolios	Dec. 31, 2010 Qualified defined benefit pension plans Fixed income security portfolios	Dec. 31, 2011 Qualified defined benefit pension plans Fair value hierarchy Level 3	Dec. 31, 2010 Qualified defined benefit pension plans Fair value hierarchy Level 3	Dec. 31, 2011 Qualified defined benefit pension plans Fair value hierarchy Level 3 Direct real estate investments	Dec. 31, 2010 Qualified defined benefit pension plans Fair value hierarchy Level 3 Direct real estate investments	Dec. 31, 2011 Other postretirement benefits	Dec. 31, 2010 Other postretirement benefits	Dec. 31, 2009 Other postretirement benefits	Dec. 31, 2011 Other postretirement benefits Principal Life general account investment	Dec. 31, 2010 Other postretirement benefits Principal Life general account investment	Dec. 31, 2011 Other postretirement benefits Fair value hierarchy Level 3	Dec. 31, 2010 Other postretirement benefits Fair value hierarchy Level 3	Dec. 31, 2011 Other postretirement benefits Fair value hierarchy Level 3 Principal Life general account investment	Dec. 31, 2010 Other postretirement benefits Fair value hierarchy Level 3 Principal Life general account investment
Change in Level 3 fair value measurement rollforward, plan assets
Fair value of plan assets at beginning of year	$ 1,429.0	$ 1,417.7	$ 1,250.3	$ 1,429.0	$ 1,417.7	$ 94.7	$ 84.7	$ 347.8	$ 331.5	$ 84.7	$ 54.0	$ 84.7	$ 54.0	$ 466.6	$ 471.7	$ 421.5	$ 42.5	$ 44.5	$ 44.5	$ 45.5	$ 44.5	$ 45.5
Actual return gains (losses) on plan assets relating to assets still held at the reporting date										1.6	10.7	1.6	10.7						3.0	4.3	3.0	4.3
Purchases, sales and settlements										1.0	20.0	1.0	20.0						(5.0)	(5.3)	(5.0)	(5.3)
Transfers in (out) of Level 3										(87.3)		(87.3)
Fair value of plan assets at end of year	$ 1,429.0	$ 1,417.7	$ 1,250.3	$ 1,429.0	$ 1,417.7	$ 94.7	$ 84.7	$ 347.8	$ 331.5		$ 84.7		$ 84.7	$ 466.6	$ 471.7	$ 421.5	$ 42.5	$ 44.5	$ 42.5	$ 44.5	$ 42.5	$ 44.5