Condensed Consolidating Financial Information - Shelf Registration Guarantor, Statements of Operations (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenues
Premiums and other considerations	$ 2,172.4	$ 2,126.1	$ 4,433.4	$ 4,826.8
Fees and other revenues	1,261.8	987.1	3,285.7	2,882.8
Net investment income (loss)	919.4	838.0	2,710.7	2,581.8
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities	(1.9)	693.2	59.3	623.7
Net other-than-temporary impairment (losses) recoveries on available-for-sale securities	(1.4)	2.6	(8.3)	(27.1)
Other-than-temporary impairment losses on fixed maturities, available-for-sale reclassified to (from) other comprehensive income	(2.2)	(19.2)	(14.2)	(32.5)
Net impairment (losses) recoveries on available-for-sale securities	(3.6)		(22.5)
Net impairment (losses) recoveries on available-for-sale securities		(16.6)		(59.6)
Net realized capital gains (losses)	(5.5)	676.6	36.8	564.1
Total revenues	4,348.1	4,627.8	10,466.6	10,855.5
Expenses
Benefits, claims and settlement expenses	2,642.1	2,504.6	5,752.4	6,020.7
Dividends to policyholders	31.9	32.6	92.6	92.7
Operating expenses	1,105.0	932.8	3,080.1	2,804.7
Total expenses	3,779.0	3,470.0	8,925.1	8,918.1
Income (loss) before income taxes	569.1	1,157.8	1,541.5	1,937.4
Income taxes (benefits)	109.1	344.6	219.5	455.8
Net income (loss)	460.0	813.2	1,322.0	1,481.6
Net income (loss) attributable to noncontrolling interest	3.7	3.0	12.0	13.0
Net income (loss) attributable to Principal Financial Group, Inc.	456.3	810.2	1,310.0	1,468.6
Net income (loss)	460.0	813.2	1,322.0	1,481.6
Other comprehensive income (loss)	(208.7)	165.1	(1,617.1)	724.9
Comprehensive income (loss)	$ 251.3	$ 978.3	(295.1)	2,206.5
Principal Financial Group, Inc. Parent Only
Revenues
Fees and other revenues				7.8
Net investment income (loss)			16.8	7.4
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities			(11.3)
Net realized capital gains (losses)			(11.3)
Total revenues			5.5	15.2
Expenses
Operating expenses			143.7	152.1
Total expenses			143.7	152.1
Income (loss) before income taxes			(138.2)	(136.9)
Income taxes (benefits)			(52.6)	(57.2)
Equity in the net income (loss) of subsidiaries			1,395.6	1,548.3
Net income (loss)			1,310.0	1,468.6
Net income (loss) attributable to Principal Financial Group, Inc.			1,310.0	1,468.6
Net income (loss)			1,310.0	1,468.6
Other comprehensive income (loss)			(1,605.4)	719.9
Comprehensive income (loss)			(295.4)	2,188.5
Principal Financial Services, Inc. Only
Revenues
Fees and other revenues			2.5	0.2
Net investment income (loss)			(4.9)	(2.0)
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities			1.3	4.5
Net realized capital gains (losses)			1.3	4.5
Total revenues			(1.1)	2.7
Expenses
Operating expenses			23.1	38.5
Total expenses			23.1	38.5
Income (loss) before income taxes			(24.2)	(35.8)
Income taxes (benefits)			6.2	(15.3)
Equity in the net income (loss) of subsidiaries			1,415.9	1,563.8
Net income (loss)			1,385.5	1,543.3
Net income (loss) attributable to Principal Financial Group, Inc.			1,385.5	1,543.3
Net income (loss)			1,385.5	1,543.3
Other comprehensive income (loss)			(1,645.1)	686.1
Comprehensive income (loss)			(259.6)	2,229.4
Principal Life Insurance Company and Other Subsidiaries Combined
Revenues
Premiums and other considerations			4,433.4	4,826.8
Fees and other revenues			3,295.3	2,881.9
Net investment income (loss)			2,687.5	2,572.0
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities			69.3	619.2
Net other-than-temporary impairment (losses) recoveries on available-for-sale securities			(8.3)	(27.1)
Other-than-temporary impairment losses on fixed maturities, available-for-sale reclassified to (from) other comprehensive income			(14.2)	(32.5)
Net impairment (losses) recoveries on available-for-sale securities			(22.5)
Net impairment (losses) recoveries on available-for-sale securities				(59.6)
Net realized capital gains (losses)			46.8	559.6
Total revenues			10,463.0	10,840.3
Expenses
Benefits, claims and settlement expenses			5,752.4	6,020.7
Dividends to policyholders			92.6	92.7
Operating expenses			2,924.2	2,620.6
Total expenses			8,769.2	8,734.0
Income (loss) before income taxes			1,693.8	2,106.3
Income taxes (benefits)			265.9	529.5
Net income (loss)			1,427.9	1,576.8
Net income (loss) attributable to noncontrolling interest			12.0	13.0
Net income (loss) attributable to Principal Financial Group, Inc.			1,415.9	1,563.8
Net income (loss)			1,427.9	1,576.8
Other comprehensive income (loss)			(1,668.9)	688.9
Comprehensive income (loss)			(241.0)	2,265.7
Eliminations, Shelf Registration Debt Guarantor
Revenues
Fees and other revenues			(12.1)	(7.1)
Net investment income (loss)			11.3	4.4
Total revenues			(0.8)	(2.7)
Expenses
Operating expenses			(10.9)	(6.5)
Total expenses			(10.9)	(6.5)
Income (loss) before income taxes			10.1	3.8
Income taxes (benefits)				(1.2)
Equity in the net income (loss) of subsidiaries			(2,811.5)	(3,112.1)
Net income (loss)			(2,801.4)	(3,107.1)
Net income (loss) attributable to Principal Financial Group, Inc.			(2,801.4)	(3,107.1)
Net income (loss)			(2,801.4)	(3,107.1)
Other comprehensive income (loss)			3,302.3	(1,370.0)
Comprehensive income (loss)			$ 500.9	$ (4,477.1)