Condensed Consolidating Financial Information - Guarantor, Statements of Operations (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenues
Premiums and other considerations									$ 7,866.6	$ 6,409.6	$ 6,217.4
Fees and other revenues									4,409.9	4,273.8	3,892.3
Net investment income (loss)									3,998.4	3,629.2	3,459.3
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities									(9.3)	(46.3)	606.0
Net other-than-temporary impairment (losses) recoveries on available-for-sale securities									(38.3)	10.6	(28.7)
Other-than-temporary impairment losses on fixed maturities, available-for-sale reclassified to (from) other comprehensive income									(5.2)	(39.7)	(53.1)
Net impairment (losses) recoveries on available-for-sale securities									(43.5)	(29.1)
Net impairment (losses) recoveries on available-for-sale securities											(81.8)
Net realized capital gains (losses)									(52.8)	(75.4)	524.2
Total revenues	$ 4,047.2	$ 4,458.4	$ 3,972.6	$ 3,743.9	$ 3,770.6	$ 4,348.1	$ 3,234.9	$ 2,883.6	16,222.1	14,237.2	14,093.2
Expenses
Benefits, claims and settlement expenses									9,905.8	8,192.5	7,822.6
Dividends to policyholders									119.1	123.6	124.6
Operating expenses									4,503.9	4,136.7	3,893.8
Total expenses	3,675.9	4,112.7	3,522.3	3,217.9	3,527.7	3,779.0	2,719.5	2,426.6	14,528.8	12,452.8	11,841.0
Income (loss) before income taxes									1,693.3	1,784.4	2,252.2
Income taxes (benefits)									249.2	230.7	(72.3)
Net income (loss)	315.3	284.6	392.1	452.1	231.7	460.0	459.5	402.5	1,444.1	1,553.7	2,324.5
Net income (loss) attributable to noncontrolling interest									49.9	7.2	14.1
Net income (loss) attributable to Principal Financial Group, Inc.	300.9	277.1	386.3	429.9	236.5	456.3	456.6	397.1	1,394.2	1,546.5	2,310.4
Net income (loss)	$ 315.3	$ 284.6	$ 392.1	$ 452.1	$ 231.7	$ 460.0	$ 459.5	$ 402.5	1,444.1	1,553.7	2,324.5
Other comprehensive income (loss)									2,606.7	(1,835.7)	844.3
Comprehensive income (loss)									4,050.8	(282.0)	3,168.8
Principal Life Insurance Company only | Legal Entities
Revenues
Premiums and other considerations									7,189.2	5,846.0	5,777.7
Fees and other revenues									2,343.9	2,174.4	2,140.9
Net investment income (loss)									2,843.7	2,655.7	2,533.1
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities									(3,509.5)	118.8	(84.5)
Net other-than-temporary impairment (losses) recoveries on available-for-sale securities									(35.6)	14.1	(28.2)
Other-than-temporary impairment losses on fixed maturities, available-for-sale reclassified to (from) other comprehensive income									(4.7)	(40.1)	(46.4)
Net impairment (losses) recoveries on available-for-sale securities									(40.3)	(26.0)
Net impairment (losses) recoveries on available-for-sale securities											(74.6)
Net realized capital gains (losses)									(3,549.8)	92.8	(159.1)
Total revenues									8,827.0	10,768.9	10,292.6
Expenses
Benefits, claims and settlement expenses									8,940.7	7,346.3	7,153.5
Dividends to policyholders									119.1	123.6	124.6
Operating expenses									2,460.0	2,320.1	2,185.4
Total expenses									11,519.8	9,790.0	9,463.5
Income (loss) before income taxes									(2,692.8)	978.9	829.1
Income taxes (benefits)									(685.5)	60.0	(540.1)
Equity in the net income (loss) of subsidiaries									3,121.0	334.7	901.5
Net income (loss)									1,113.7	1,253.6	2,270.7
Net income (loss) attributable to Principal Financial Group, Inc.									1,113.7	1,253.6	2,270.7
Net income (loss)									1,113.7	1,253.6	2,270.7
Other comprehensive income (loss)									2,564.6	(1,548.5)	611.6
Comprehensive income (loss)									3,678.3	(294.9)	2,882.3
Income (loss) from discontinued operations, net of related income taxes											35.5
Eliminations, Notes Guarantor | Eliminations
Revenues
Fees and other revenues									(395.9)	(387.6)	(419.8)
Net investment income (loss)									(1,088.6)	(1,244.9)	(2,264.4)
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities									(0.6)	18.2	(0.7)
Net impairment (losses) recoveries on available-for-sale securities										0.0
Net realized capital gains (losses)									(0.6)	18.2	(0.7)
Total revenues									(1,485.1)	(1,614.3)	(2,684.9)
Expenses
Benefits, claims and settlement expenses									(11.3)	(10.9)	(10.8)
Operating expenses									(373.2)	(358.0)	(352.9)
Total expenses									(384.5)	(368.9)	(363.7)
Income (loss) before income taxes									(1,100.6)	(1,245.4)	(2,321.2)
Income taxes (benefits)									(1.3)	(1.8)	28.3
Equity in the net income (loss) of subsidiaries									(1,534.6)	(1,657.7)	(2,490.9)
Net income (loss)									(2,633.9)	(2,901.3)	(4,840.4)
Net income (loss) attributable to Principal Financial Group, Inc.									(2,633.9)	(2,901.3)	(4,840.4)
Net income (loss)									(2,633.9)	(2,901.3)	(4,840.4)
Other comprehensive income (loss)									(5,182.8)	3,465.8	(1,485.2)
Comprehensive income (loss)									(7,816.7)	564.5	(6,325.6)
Principal Financial Services, Inc. and other subsidiaries combined | Legal Entities
Revenues
Premiums and other considerations									677.4	563.6	439.7
Fees and other revenues									2,461.9	2,487.0	2,163.4
Net investment income (loss)									2,223.9	2,192.6	3,180.9
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities									3,488.5	(175.1)	691.8
Net other-than-temporary impairment (losses) recoveries on available-for-sale securities									(2.7)	(3.5)	(0.5)
Other-than-temporary impairment losses on fixed maturities, available-for-sale reclassified to (from) other comprehensive income									(0.5)	0.4	(6.7)
Net impairment (losses) recoveries on available-for-sale securities									(3.2)	(3.1)
Net impairment (losses) recoveries on available-for-sale securities											(7.2)
Net realized capital gains (losses)									3,485.3	(178.2)	684.6
Total revenues									8,848.5	5,065.0	6,468.6
Expenses
Benefits, claims and settlement expenses									976.4	857.1	679.9
Operating expenses									2,206.2	1,982.7	1,823.2
Total expenses									3,182.6	2,839.8	2,503.1
Income (loss) before income taxes									5,665.9	2,225.2	3,965.5
Income taxes (benefits)									980.2	235.0	376.4
Equity in the net income (loss) of subsidiaries									(3,115.6)	(335.3)	(1,005.3)
Net income (loss)									1,570.1	1,654.9	2,583.8
Net income (loss) attributable to noncontrolling interest									49.9	7.2	14.1
Net income (loss) attributable to Principal Financial Group, Inc.									1,520.2	1,647.7	2,569.7
Net income (loss)									1,570.1	1,654.9	2,583.8
Other comprehensive income (loss)									2,654.0	(1,928.5)	877.6
Comprehensive income (loss)									4,224.1	(273.6)	3,461.4
Principal Financial Group, Inc. parent only | Legal Entities
Revenues
Fees and other revenues											7.8
Net investment income (loss)									19.4	25.8	9.7
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities									12.3	(8.2)	(0.6)
Net impairment (losses) recoveries on available-for-sale securities										0.0
Net realized capital gains (losses)									12.3	(8.2)	(0.6)
Total revenues									31.7	17.6	16.9
Expenses
Operating expenses									210.9	191.9	238.1
Total expenses									210.9	191.9	238.1
Income (loss) before income taxes									(179.2)	(174.3)	(221.2)
Income taxes (benefits)									(44.2)	(62.5)	63.1
Equity in the net income (loss) of subsidiaries									1,529.2	1,658.3	2,594.7
Net income (loss)									1,394.2	1,546.5	2,310.4
Net income (loss) attributable to Principal Financial Group, Inc.									1,394.2	1,546.5	2,310.4
Net income (loss)									1,394.2	1,546.5	2,310.4
Other comprehensive income (loss)									2,570.9	(1,824.5)	840.3
Comprehensive income (loss)									$ 3,965.1	$ (278.0)	$ 3,150.7