Condensed Consolidating Financial Information - Guarantor, Statements of Operations (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Revenues
Premiums and other considerations	$ 2,126.1	$ 1,002.7	$ 4,826.8	$ 3,561.1
Fees and other revenues	987.1	915.0	2,882.8	2,687.7
Net investment income (loss)	838.0	856.1	2,581.8	2,417.0
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities	693.2	55.5	623.7	283.9
Net other-than-temporary impairment (losses) recoveries on available-for-sale securities	2.6	(22.0)	(27.1)	(81.0)
Other-than-temporary impairment losses on fixed maturities, available-for-sale reclassified to (from) other comprehensive income	(19.2)	10.7	(32.5)	11.6
Net impairment (losses) recoveries on available-for-sale securities	(16.6)	(11.3)	(59.6)	(69.4)
Net realized capital gains (losses)	676.6	44.2	564.1	214.5
Total revenues	4,627.8	2,818.0	10,855.5	8,880.3
Expenses
Benefits, claims and settlement expenses	2,504.6	1,482.2	6,020.7	4,800.6
Dividends to policyholders	32.6	45.5	92.7	122.3
Operating expenses	932.8	919.6	2,804.7	2,747.0
Total expenses	3,470.0	2,447.3	8,918.1	7,669.9
Income (loss) before income taxes	1,157.8	370.7	1,937.4	1,210.4
Income taxes (benefits)	344.6	43.3	455.8	186.8
Net income (loss)	813.2	327.4	1,481.6	1,023.6
Net income (loss) attributable to noncontrolling interest	3.0	19.2	13.0	25.1
Net income (loss) attributable to Principal Financial Group, Inc.	810.2	308.2	1,468.6	998.5
Net income (loss)	813.2	327.4	1,481.6	1,023.6
Other comprehensive income (loss)	165.1	110.7	724.9	989.0
Comprehensive income (loss)	$ 978.3	$ 438.1	2,206.5	2,012.6
Principal Financial Group, Inc. Parent Only
Revenues
Fees and other revenues			7.8
Net investment income (loss)			7.4	2.3
Total revenues			15.2	2.3
Expenses
Operating expenses			152.1	170.5
Total expenses			152.1	170.5
Income (loss) before income taxes			(136.9)	(168.2)
Income taxes (benefits)			(57.2)	(68.2)
Equity in the net income (loss) of subsidiaries			1,548.3	1,098.5
Net income (loss)			1,468.6	998.5
Net income (loss) attributable to Principal Financial Group, Inc.			1,468.6	998.5
Net income (loss)			1,468.6	998.5
Other comprehensive income (loss)			719.9	978.6
Comprehensive income (loss)			2,188.5	1,977.1
Principal Life Insurance Company Only
Revenues
Premiums and other considerations			4,512.5	3,291.6
Fees and other revenues			1,584.2	1,455.6
Net investment income (loss)			1,883.5	1,714.5
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities			(119.2)	810.3
Net other-than-temporary impairment (losses) recoveries on available-for-sale securities			(27.8)	(75.4)
Other-than-temporary impairment losses on fixed maturities, available-for-sale reclassified to (from) other comprehensive income			(26.1)	8.9
Net impairment (losses) recoveries on available-for-sale securities			(53.9)	(66.5)
Net realized capital gains (losses)			(173.1)	743.8
Total revenues			7,807.1	7,205.5
Expenses
Benefits, claims and settlement expenses			5,529.9	4,297.8
Dividends to policyholders			92.7	122.3
Operating expenses			1,570.2	1,616.7
Total expenses			7,192.8	6,036.8
Income (loss) before income taxes			614.3	1,168.7
Income taxes (benefits)			34.1	272.2
Equity in the net income (loss) of subsidiaries			566.5	(107.5)
Net income (loss)			1,146.7	789.0
Net income (loss) attributable to Principal Financial Group, Inc.			1,146.7	789.0
Net income (loss)			1,146.7	789.0
Other comprehensive income (loss)			495.8	781.9
Comprehensive income (loss)			1,642.5	1,570.9
Income (loss) from discontinued operations, net of related income taxes			35.5	94.1
Principal Financial Services, Inc. and Other Subsidiaries Combined
Revenues
Premiums and other considerations			314.3	269.5
Fees and other revenues			1,607.2	1,519.3
Net investment income (loss)			1,836.6	1,442.2
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities			743.6	(526.4)
Net other-than-temporary impairment (losses) recoveries on available-for-sale securities			0.7	(5.6)
Other-than-temporary impairment losses on fixed maturities, available-for-sale reclassified to (from) other comprehensive income			(6.4)	2.7
Net impairment (losses) recoveries on available-for-sale securities			(5.7)	(2.9)
Net realized capital gains (losses)			737.9	(529.3)
Total revenues			4,496.0	2,701.7
Expenses
Benefits, claims and settlement expenses			498.9	511.1
Operating expenses			1,345.8	1,201.8
Total expenses			1,844.7	1,712.9
Income (loss) before income taxes			2,651.3	988.8
Income taxes (benefits)			481.9	(15.6)
Equity in the net income (loss) of subsidiaries			(613.1)	114.2
Net income (loss)			1,556.3	1,118.6
Net income (loss) attributable to noncontrolling interest			13.0	25.1
Net income (loss) attributable to Principal Financial Group, Inc.			1,543.3	1,093.5
Net income (loss)			1,556.3	1,118.6
Other comprehensive income (loss)			750.1	1,001.8
Comprehensive income (loss)			2,306.4	2,120.4
Eliminations, Notes Guarantor
Revenues
Fees and other revenues			(316.4)	(287.2)
Net investment income (loss)			(1,145.7)	(742.0)
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities			(0.7)
Net realized capital gains (losses)			(0.7)
Total revenues			(1,462.8)	(1,029.2)
Expenses
Benefits, claims and settlement expenses			(8.1)	(8.3)
Operating expenses			(263.4)	(242.0)
Total expenses			(271.5)	(250.3)
Income (loss) before income taxes			(1,191.3)	(778.9)
Income taxes (benefits)			(3.0)	(1.6)
Equity in the net income (loss) of subsidiaries			(1,501.7)	(1,105.2)
Net income (loss)			(2,690.0)	(1,882.5)
Net income (loss) attributable to Principal Financial Group, Inc.			(2,690.0)	(1,882.5)
Net income (loss)			(2,690.0)	(1,882.5)
Other comprehensive income (loss)			(1,240.9)	(1,773.3)
Comprehensive income (loss)			$ (3,930.9)	$ (3,655.8)