Segment Information - Reconciliation of Segment Operating Revenues and Earnings to Consolidated (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Revenue And Profit (Loss) From Segments To Consolidated
Net realized capital gains (losses), net of related revenue adjustments									$ 472.2	$ 80.9	$ (162.7)
Total revenues	$ 3,237.7	$ 4,627.8	$ 3,178.3	$ 3,049.4	$ 3,513.8	$ 2,818.0	$ 3,025.7	$ 3,036.6	14,093.2	12,394.1	11,964.4
Pre-tax net realized capital gains (losses), as adjusted									520.3	46.3	(170.7)
Income (loss) before income taxes									2,252.2	1,591.7	1,430.8
Impact of court ruling on uncertain tax positions											15.1
Pre-tax gains (losses) associated with exited group medical insurance business that does not qualify for discontinued operations											(3.4)
Operating Segments
Operating Revenue And Profit (Loss) From Segments To Consolidated
Operating revenues									13,703.3	12,380.8	12,121.1
Pre-tax operating earnings (losses)									1,873.7	1,668.7	1,637.1
Operating Segments | Retirement and Income Solutions
Operating Revenue And Profit (Loss) From Segments To Consolidated
Operating revenues									7,165.6	6,150.7	6,166.9
Pre-tax operating earnings (losses)									899.8	794.5	740.1
Inter-segment revenues									402.8	373.3	424.5
Operating Segments | Retirement and Income Solutions | Retirement and Income Solutions - Fee
Operating Revenue And Profit (Loss) From Segments To Consolidated
Operating revenues									1,821.0	1,743.2	1,774.0
Operating Segments | Retirement and Income Solutions | Retirement and Income Solutions - Spread
Operating Revenue And Profit (Loss) From Segments To Consolidated
Operating revenues									5,344.6	4,407.5	4,392.9
Operating Segments | Principal Global Investors
Operating Revenue And Profit (Loss) From Segments To Consolidated
Operating revenues									1,444.4	1,387.1	1,343.5
Pre-tax operating earnings (losses)									469.7	443.8	388.5
Inter-segment revenues									250.9	235.7	220.6
Operating Segments | Principal International
Operating Revenue And Profit (Loss) From Segments To Consolidated
Operating revenues									1,251.5	1,252.0	1,220.6
Pre-tax operating earnings (losses)									330.0	288.1	271.3
Operating Segments | U.S. Insurance Solutions
Operating Revenue And Profit (Loss) From Segments To Consolidated
Operating revenues									3,902.6	3,637.3	3,440.6
Pre-tax operating earnings (losses)									384.7	361.2	429.5
Operating Segments | U.S. Insurance Solutions | Specialty benefits insurance
Operating Revenue And Profit (Loss) From Segments To Consolidated
Operating revenues									2,171.8	2,011.4	1,868.1
Operating Segments | U.S. Insurance Solutions | Individual life insurance
Operating Revenue And Profit (Loss) From Segments To Consolidated
Operating revenues									1,731.0	1,626.1	1,572.7
Operating Segments | U.S. Insurance Solutions | U.S. Insurance Solutions Eliminations
Operating Revenue And Profit (Loss) From Segments To Consolidated
Operating revenues									(0.2)	(0.2)	(0.2)
Operating Segments | Corporate
Operating Revenue And Profit (Loss) From Segments To Consolidated
Operating revenues									(60.8)	(46.3)	(50.5)
Pre-tax operating earnings (losses)									(210.5)	(218.9)	(192.3)
Reconciling Items
Operating Revenue And Profit (Loss) From Segments To Consolidated
Net realized capital gains (losses), net of related revenue adjustments									472.2	80.9	(162.7)
Revenue adjustments related to equity method investments									(82.3)	(67.6)	(55.5)
Other income on a tax indemnification											60.2
Exited group medical insurance business											1.3
Pre-tax net realized capital gains (losses), as adjusted									520.3	46.3	(170.7)
Pre-tax other adjustments									(70.0)	(86.4)	11.7
Earnings adjustments related to equity method investments and noncontrolling interest									$ (71.8)	$ (36.9)	$ (47.3)