Condensed Consolidating Financial Information - Shelf Registration Guarantor, Statements of Operations (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues
Premiums and other considerations									$ 6,409.6	$ 6,217.4	$ 5,299.1
Fees and other revenues									4,273.8	3,892.3	3,627.4
Net investment income (loss)									3,629.2	3,459.3	3,296.5
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities									(46.3)	606.0	269.5
Net other-than-temporary impairment (losses) recoveries on available-for-sale securities									10.6	(28.7)	(98.8)
Other-than-temporary impairment losses on fixed maturities, available-for-sale reclassified to (from) other comprehensive income									(39.7)	(53.1)	0.4
Net impairment (losses) recoveries on available-for-sale securities									(29.1)
Net impairment (losses) recoveries on available-for-sale securities										(81.8)	(98.4)
Net realized capital gains (losses)									(75.4)	524.2	171.1
Total revenues	$ 3,770.6	$ 4,348.1	$ 3,234.9	$ 2,883.6	$ 3,237.7	$ 4,627.8	$ 3,178.3	$ 3,049.4	14,237.2	14,093.2	12,394.1
Expenses
Benefits, claims and settlement expenses									8,192.5	7,822.6	6,913.2
Dividends to policyholders									123.6	124.6	156.6
Operating expenses									4,136.7	3,893.8	3,732.6
Total expenses	3,527.7	3,779.0	2,719.5	2,426.6	2,922.9	3,470.0	2,812.7	2,635.4	12,452.8	11,841.0	10,802.4
Income (loss) before income taxes									1,784.4	2,252.2	1,591.7
Income taxes (benefits)									230.7	(72.3)	229.9
Net income (loss)	231.7	460.0	459.5	402.5	842.9	813.2	314.8	353.6	1,553.7	2,324.5	1,361.8
Net income (loss) attributable to noncontrolling interest									7.2	14.1	45.3
Net income (loss) attributable to Principal Financial Group, Inc.	236.5	456.3	456.6	397.1	841.8	810.2	309.5	348.9	1,546.5	2,310.4	1,316.5
Net income (loss)	$ 231.7	$ 460.0	$ 459.5	$ 402.5	$ 842.9	$ 813.2	$ 314.8	$ 353.6	1,553.7	2,324.5	1,361.8
Other comprehensive income (loss)									(1,835.7)	844.3	221.0
Comprehensive income (loss)									(282.0)	3,168.8	1,582.8
Principal Financial Group, Inc. Parent Only
Revenues
Fees and other revenues										7.8
Net investment income (loss)									25.8	9.7	3.4
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities									(8.2)	(0.6)
Net realized capital gains (losses)									(8.2)	(0.6)
Total revenues									17.6	16.9	3.4
Expenses
Operating expenses									191.9	238.1	312.3
Total expenses									191.9	238.1	312.3
Income (loss) before income taxes									(174.3)	(221.2)	(308.9)
Income taxes (benefits)									(62.5)	63.1	(134.9)
Equity in the net income (loss) of subsidiaries									1,658.3	2,594.7	1,490.5
Net income (loss)									1,546.5	2,310.4	1,316.5
Net income (loss) attributable to Principal Financial Group, Inc.									1,546.5	2,310.4	1,316.5
Net income (loss)									1,546.5	2,310.4	1,316.5
Other comprehensive income (loss)									(1,824.5)	840.3	218.8
Comprehensive income (loss)									(278.0)	3,150.7	1,535.3
Principal Financial Services, Inc. Only
Revenues
Fees and other revenues									2.7	0.2	0.9
Net investment income (loss)									(6.7)	(2.8)	25.2
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities									(3.5)	4.2	(4.5)
Net realized capital gains (losses)									(3.5)	4.2	(4.5)
Total revenues									(7.5)	1.6	21.6
Expenses
Operating expenses									29.1	48.4	9.7
Total expenses									29.1	48.4	9.7
Income (loss) before income taxes									(36.6)	(46.8)	11.9
Income taxes (benefits)									0.2	115.4	(24.3)
Equity in the net income (loss) of subsidiaries									1,684.5	2,731.9	1,447.3
Net income (loss)									1,647.7	2,569.7	1,483.5
Net income (loss) attributable to Principal Financial Group, Inc.									1,647.7	2,569.7	1,483.5
Net income (loss)									1,647.7	2,569.7	1,483.5
Other comprehensive income (loss)									(1,839.1)	809.8	174.3
Comprehensive income (loss)									(191.4)	3,379.5	1,657.8
Principal Life Insurance Company and Other Subsidiaries Combined
Revenues
Premiums and other considerations									6,409.6	6,217.4	5,299.1
Fees and other revenues									4,286.4	3,894.7	3,634.0
Net investment income (loss)									3,598.0	3,396.5	3,260.4
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities									(34.6)	602.4	273.9
Net other-than-temporary impairment (losses) recoveries on available-for-sale securities									10.6	(28.7)	(98.8)
Other-than-temporary impairment losses on fixed maturities, available-for-sale reclassified to (from) other comprehensive income									(39.7)	(53.1)	0.4
Net impairment (losses) recoveries on available-for-sale securities									(29.1)
Net impairment (losses) recoveries on available-for-sale securities										(81.8)	(98.4)
Net realized capital gains (losses)									(63.7)	520.6	175.5
Total revenues									14,230.3	14,029.2	12,369.0
Expenses
Benefits, claims and settlement expenses									8,192.5	7,822.6	6,913.2
Dividends to policyholders									123.6	124.6	156.6
Operating expenses									3,929.5	3,616.9	3,417.5
Total expenses									12,245.6	11,564.1	10,487.3
Income (loss) before income taxes									1,984.7	2,465.1	1,881.7
Income taxes (benefits)									293.0	(280.9)	389.1
Net income (loss)									1,691.7	2,746.0	1,492.6
Net income (loss) attributable to noncontrolling interest									7.2	14.1	45.3
Net income (loss) attributable to Principal Financial Group, Inc.									1,684.5	2,731.9	1,447.3
Net income (loss)									1,691.7	2,746.0	1,492.6
Other comprehensive income (loss)									(1,856.4)	801.8	191.7
Comprehensive income (loss)									(164.7)	3,547.8	1,684.3
Eliminations, Shelf Registration Debt Guarantor
Revenues
Fees and other revenues									(15.3)	(10.4)	(7.5)
Net investment income (loss)									12.1	55.9	7.5
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities											0.1
Net realized capital gains (losses)											0.1
Total revenues									(3.2)	45.5	0.1
Expenses
Operating expenses									(13.8)	(9.6)	(6.9)
Total expenses									(13.8)	(9.6)	(6.9)
Income (loss) before income taxes									10.6	55.1	7.0
Income taxes (benefits)										30.1
Equity in the net income (loss) of subsidiaries									(3,342.8)	(5,326.6)	(2,937.8)
Net income (loss)									(3,332.2)	(5,301.6)	(2,930.8)
Net income (loss) attributable to Principal Financial Group, Inc.									(3,332.2)	(5,301.6)	(2,930.8)
Net income (loss)									(3,332.2)	(5,301.6)	(2,930.8)
Other comprehensive income (loss)									3,684.3	(1,607.6)	(363.8)
Comprehensive income (loss)									$ 352.1	$ (6,909.2)	$ (3,294.6)