Condensed Consolidating Financial Information - Shelf Registration Guarantor, Statements of Operations (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenues
Premiums and other considerations									$ 6,037.4	$ 7,866.6	$ 6,409.6
Fees and other revenues									4,511.1	4,409.9	4,273.8
Net investment income (loss)									3,890.6	3,998.4	3,629.2
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities										(9.3)	(46.3)
Net other-than-temporary impairment (losses) recoveries on available-for-sale securities										(38.3)	10.6
Other-than-temporary impairment losses on fixed maturities, available-for-sale reclassified to (from) other comprehensive income										(5.2)	(39.7)
Net impairment (losses) recoveries on available-for-sale securities										(43.5)	(29.1)
Net realized capital gains (losses)									302.6	(52.8)	(75.4)
Total revenues	$ 3,765.1	$ 3,310.7	$ 3,114.6	$ 4,551.3	$ 4,047.2	$ 4,458.4	$ 3,972.6	$ 3,743.9	14,741.7	16,222.1	14,237.2
Expenses
Benefits, claims and settlement expenses									8,281.5	9,905.8	8,192.5
Dividends to policyholders									120.2	119.1	123.6
Operating expenses									4,646.5	4,503.9	4,136.7
Total expenses	3,173.5	3,035.6	2,616.0	4,223.1	3,675.9	4,112.7	3,522.3	3,217.9	13,048.2	14,528.8	12,452.8
Income (loss) before income taxes									1,693.5	1,693.3	1,784.4
Income taxes (benefits)									265.0	249.2	230.7
Net income (loss)	491.5	235.9	416.1	285.0	315.3	284.6	392.1	452.1	1,428.5	1,444.1	1,553.7
Net income attributable to noncontrolling interest									32.7	49.9	7.2
Net income attributable to PFG	472.6	236.0	398.3	288.9	300.9	277.1	386.3	429.9	1,395.8	1,394.2	1,546.5
Net income (loss)	$ 491.5	$ 235.9	$ 416.1	$ 285.0	$ 315.3	$ 284.6	$ 392.1	$ 452.1	1,428.5	1,444.1	1,553.7
Other comprehensive income									1,347.0	2,606.7	(1,835.7)
Comprehensive income (loss)									2,775.5	4,050.8	(282.0)
Principal Financial Group, Inc. parent only | Legal Entities
Revenues
Net investment income (loss)									13.3	19.4	25.8
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities										12.3	(8.2)
Net realized capital gains (losses)									7.0	12.3	(8.2)
Total revenues									20.3	31.7	17.6
Expenses
Operating expenses									200.0	210.9	191.9
Total expenses									200.0	210.9	191.9
Income (loss) before income taxes									(179.7)	(179.2)	(174.3)
Income taxes (benefits)									(46.1)	(44.2)	(62.5)
Equity in the net income (loss) of subsidiaries									1,529.4	1,529.2	1,658.3
Net income (loss)									1,395.8	1,394.2	1,546.5
Net income attributable to PFG									1,395.8	1,394.2	1,546.5
Net income (loss)									1,395.8	1,394.2	1,546.5
Other comprehensive income									1,206.5	2,570.9	(1,824.5)
Comprehensive income (loss)									2,602.3	3,965.1	(278.0)
Principal Financial Services, Inc. only | Legal Entities
Revenues
Fees and other revenues									341.3	205.9	2.7
Net investment income (loss)									(1.4)	(6.5)	(6.7)
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities										17.5	(3.5)
Net realized capital gains (losses)									32.8	17.5	(3.5)
Total revenues									372.7	216.9	(7.5)
Expenses
Operating expenses									548.7	368.2	29.1
Total expenses									548.7	368.2	29.1
Income (loss) before income taxes									(176.0)	(151.3)	(36.6)
Income taxes (benefits)									(45.8)	(11.8)	0.2
Equity in the net income (loss) of subsidiaries									1,649.4	1,659.7	1,684.5
Net income (loss)									1,519.2	1,520.2	1,647.7
Net income attributable to PFG									1,519.2	1,520.2	1,647.7
Net income (loss)									1,519.2	1,520.2	1,647.7
Other comprehensive income									1,228.6	2,530.9	(1,839.1)
Comprehensive income (loss)									2,747.8	4,051.1	(191.4)
Principal Life Insurance Company and other subsidiaries combined | Legal Entities
Revenues
Premiums and other considerations									6,037.4	7,866.6	6,409.6
Fees and other revenues									4,182.9	4,215.5	4,286.4
Net investment income (loss)									3,867.1	3,975.1	3,598.0
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities										(38.9)	(34.6)
Net other-than-temporary impairment (losses) recoveries on available-for-sale securities										(38.3)	10.6
Other-than-temporary impairment losses on fixed maturities, available-for-sale reclassified to (from) other comprehensive income										(5.2)	(39.7)
Net impairment (losses) recoveries on available-for-sale securities										(43.5)	(29.1)
Net realized capital gains (losses)									262.9	(82.4)	(63.7)
Total revenues									14,350.3	15,974.8	14,230.3
Expenses
Benefits, claims and settlement expenses									8,281.5	9,905.8	8,192.5
Dividends to policyholders									120.2	119.1	123.6
Operating expenses									3,909.6	3,935.1	3,929.5
Total expenses									12,311.3	13,960.0	12,245.6
Income (loss) before income taxes									2,039.0	2,014.8	1,984.7
Income taxes (benefits)									356.9	305.2	293.0
Net income (loss)									1,682.1	1,709.6	1,691.7
Net income attributable to noncontrolling interest									32.7	49.9	7.2
Net income attributable to PFG									1,649.4	1,659.7	1,684.5
Net income (loss)									1,682.1	1,709.6	1,691.7
Other comprehensive income									1,352.7	2,562.2	(1,856.4)
Comprehensive income (loss)									3,034.8	4,271.8	(164.7)
Eliminations, Shelf Registration Debt Guarantor | Eliminations
Revenues
Fees and other revenues									(13.1)	(11.5)	(15.3)
Net investment income (loss)									11.6	10.4	12.1
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities										(0.2)
Net realized capital gains (losses)									(0.1)	(0.2)
Total revenues									(1.6)	(1.3)	(3.2)
Expenses
Operating expenses									(11.8)	(10.3)	(13.8)
Total expenses									(11.8)	(10.3)	(13.8)
Income (loss) before income taxes									10.2	9.0	10.6
Equity in the net income (loss) of subsidiaries									(3,178.8)	(3,188.9)	(3,342.8)
Net income (loss)									(3,168.6)	(3,179.9)	(3,332.2)
Net income attributable to PFG									(3,168.6)	(3,179.9)	(3,332.2)
Net income (loss)									(3,168.6)	(3,179.9)	(3,332.2)
Other comprehensive income									(2,440.8)	(5,057.3)	3,684.3
Comprehensive income (loss)									$ (5,609.4)	$ (8,237.2)	$ 352.1