Condensed Consolidating Financial Information - Guarantor, Statements of Operations (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Revenues
Premiums and other considerations	$ 1,153.1	$ 1,932.9	$ 3,444.1	$ 3,657.8
Fees and other revenues	1,044.3	1,006.6	2,189.6	1,980.1
Net investment income	944.6	1,037.1	1,928.2	1,998.1
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities		13.3	104.0	104.6
Net other-than-temporary impairment losses on available-for-sale securities		(17.0)		(26.6)
Other-than-temporary impairment losses on fixed maturities, available-for-sale reclassified to (from) other comprehensive income		(0.3)		2.5
Net impairment (losses) recoveries on available-for-sale securities		(17.3)		(24.1)
Net realized capital gains (losses)	(27.4)	(4.0)	104.0	80.5
Total revenues	3,114.6	3,972.6	7,665.9	7,716.5
Expenses
Benefits, claims and settlement expenses	1,601.3	2,446.1	4,460.0	4,641.2
Dividends to policyholders	31.4	29.9	60.3	60.0
Operating expenses	983.3	1,046.3	2,318.8	2,039.0
Total expenses	2,616.0	3,522.3	6,839.1	6,740.2
Income (loss) before income taxes	498.6	450.3	826.8	976.3
Income taxes (benefits)	82.5	58.2	125.7	132.1
Net income (loss)	416.1	392.1	701.1	844.2
Net income attributable to noncontrolling interest	17.8	5.8	13.9	28.0
Net income attributable to PFG	398.3	386.3	687.2	816.2
Net income (loss)	416.1	392.1	701.1	844.2
Other comprehensive income	2,048.1	1,072.3	401.8	2,219.0
Comprehensive income (loss)	$ 2,464.2	$ 1,464.4	1,102.9	3,063.2
Principal Financial Group, Inc. parent only | Legal Entities
Revenues
Net investment income			6.7	10.9
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities			4.9	9.2
Net realized capital gains (losses)				9.2
Total revenues			11.6	20.1
Expenses
Operating expenses			98.4	110.1
Total expenses			98.4	110.1
Income (loss) before income taxes			(86.8)	(90.0)
Income taxes (benefits)			(21.6)	(22.3)
Equity in the net income (loss) of subsidiaries			752.4	883.9
Net income (loss)			687.2	816.2
Net income attributable to PFG			687.2	816.2
Net income (loss)			687.2	816.2
Other comprehensive income			351.8	2,201.1
Comprehensive income (loss)			1,039.0	3,017.3
Principal Life Insurance Company only | Legal Entities
Revenues
Premiums and other considerations			3,200.9	3,293.1
Fees and other revenues			1,113.9	1,102.5
Net investment income			1,408.8	1,419.2
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities			3,444.7	411.8
Net other-than-temporary impairment losses on available-for-sale securities				(24.1)
Other-than-temporary impairment losses on fixed maturities, available-for-sale reclassified to (from) other comprehensive income				2.6
Net impairment (losses) recoveries on available-for-sale securities				(21.5)
Net realized capital gains (losses)				390.3
Total revenues			9,168.3	6,205.1
Expenses
Benefits, claims and settlement expenses			4,081.5	4,116.2
Dividends to policyholders			60.3	60.0
Operating expenses			1,268.7	1,189.3
Total expenses			5,410.5	5,365.5
Income (loss) before income taxes			3,757.8	839.6
Income taxes (benefits)			742.7	110.7
Equity in the net income (loss) of subsidiaries			(2,420.4)	(131.6)
Net income (loss)			594.7	597.3
Net income attributable to PFG			594.7	597.3
Net income (loss)			594.7	597.3
Other comprehensive income			650.0	2,011.1
Comprehensive income (loss)			1,244.7	2,608.4
Principal Financial Services, Inc. and other subsidiaries combined | Legal Entities
Revenues
Premiums and other considerations			243.2	364.7
Fees and other revenues			1,269.6	1,067.7
Net investment income			1,094.0	1,151.0
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities			(3,348.3)	(316.4)
Net other-than-temporary impairment losses on available-for-sale securities				(2.5)
Other-than-temporary impairment losses on fixed maturities, available-for-sale reclassified to (from) other comprehensive income				(0.1)
Net impairment (losses) recoveries on available-for-sale securities				(2.6)
Net realized capital gains (losses)				(319.0)
Total revenues			(741.5)	2,264.4
Expenses
Benefits, claims and settlement expenses			384.0	530.7
Operating expenses			1,135.5	917.0
Total expenses			1,519.5	1,447.7
Income (loss) before income taxes			(2,261.0)	816.7
Income taxes (benefits)			(595.1)	44.8
Equity in the net income (loss) of subsidiaries			2,426.8	134.5
Net income (loss)			760.9	906.4
Net income attributable to noncontrolling interest			13.9	25.6
Net income attributable to PFG			747.0	880.8
Net income (loss)			760.9	906.4
Other comprehensive income			374.0	2,227.8
Comprehensive income (loss)			1,134.9	3,134.2
Eliminations, Notes Guarantor | Eliminations
Revenues
Fees and other revenues			(193.9)	(190.1)
Net investment income			(581.3)	(583.0)
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities			2.7
Total revenues			(772.5)	(773.1)
Expenses
Benefits, claims and settlement expenses			(5.5)	(5.7)
Operating expenses			(183.8)	(177.4)
Total expenses			(189.3)	(183.1)
Income (loss) before income taxes			(583.2)	(590.0)
Income taxes (benefits)			(0.3)	(1.1)
Equity in the net income (loss) of subsidiaries			(758.8)	(886.8)
Net income (loss)			(1,341.7)	(1,475.7)
Net income attributable to noncontrolling interest				2.4
Net income attributable to PFG			(1,341.7)	(1,478.1)
Net income (loss)			(1,341.7)	(1,475.7)
Other comprehensive income			(974.0)	(4,221.0)
Comprehensive income (loss)			$ (2,315.7)	$ (5,696.7)