Condensed Consolidating Financial Information - Shelf Registration Guarantor, Statements of Operations (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Revenues
Premiums and other considerations	$ 1,932.9	$ 1,265.8	$ 3,657.8	$ 2,261.0
Fees and other revenues	1,006.6	1,012.6	1,980.1	2,023.9
Net investment income (loss)	1,037.1	889.1	1,998.1	1,791.3
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities	13.3	76.4	104.6	61.2
Net other-than-temporary impairment (losses) recoveries on fixed maturities, available-for-sale	(17.0)	(8.2)	(26.6)	(6.9)
Other-than-temporary impairment losses on fixed maturities, available-for-sale reclassified to (from) other comprehensive income	(0.3)	(0.8)	2.5	(12.0)
Net impairment (losses) recoveries on fixed maturities, available-for-sale	(17.3)	(9.0)	(24.1)	(18.9)
Net realized capital gains (losses)	(4.0)	67.4	80.5	42.3
Total revenues	3,972.6	3,234.9	7,716.5	6,118.5
Expenses
Benefits, claims and settlement expenses	2,446.1	1,699.2	4,641.2	3,110.3
Dividends to policyholders	29.9	30.2	60.0	60.7
Operating expenses	1,046.3	990.1	2,039.0	1,975.1
Total expenses	3,522.3	2,719.5	6,740.2	5,146.1
Income (loss) before income taxes	450.3	515.4	976.3	972.4
Income taxes (benefits)	58.2	55.9	132.1	110.4
Net income (loss)	392.1	459.5	844.2	862.0
Net income (loss) attributable to noncontrolling interest	5.8	2.9	28.0	8.3
Net income (loss) attributable to Principal Financial Group, Inc.	386.3	456.6	816.2	853.7
Net income (loss)	392.1	459.5	844.2	862.0
Other comprehensive income (loss)	1,072.3	(672.3)	2,219.0	(1,408.4)
Comprehensive income (loss)	$ 1,464.4	$ (212.8)	3,063.2	(546.4)
Principal Financial Group, Inc. Parent Only | Legal Entities
Revenues
Net investment income (loss)			10.9	11.8
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities			9.2	(8.6)
Net realized capital gains (losses)			9.2	(8.6)
Total revenues			20.1	3.2
Expenses
Operating expenses			110.1	95.2
Total expenses			110.1	95.2
Income (loss) before income taxes			(90.0)	(92.0)
Income taxes (benefits)			(22.3)	(41.0)
Equity in the net income (loss) of subsidiaries			883.9	904.7
Net income (loss)			816.2	853.7
Net income (loss) attributable to Principal Financial Group, Inc.			816.2	853.7
Net income (loss)			816.2	853.7
Other comprehensive income (loss)			2,201.1	(1,398.2)
Comprehensive income (loss)			3,017.3	(544.5)
Principal Financial Services, Inc. Only | Legal Entities
Revenues
Fees and other revenues			0.3	0.3
Net investment income (loss)			(4.1)	(3.0)
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities			5.9	(0.5)
Net realized capital gains (losses)			5.9	(0.5)
Total revenues			2.1	(3.2)
Expenses
Operating expenses			60.1	16.7
Total expenses			60.1	16.7
Income (loss) before income taxes			(58.0)	(19.9)
Income taxes (benefits)			(11.8)	(2.9)
Equity in the net income (loss) of subsidiaries			927.0	916.2
Net income (loss)			880.8	899.2
Net income (loss) attributable to Principal Financial Group, Inc.			880.8	899.2
Net income (loss)			880.8	899.2
Other comprehensive income (loss)			2,127.0	(1,424.8)
Comprehensive income (loss)			3,007.8	(525.6)
Principal Life Insurance Company and Other Subsidiaries Combined | Legal Entities
Revenues
Premiums and other considerations			3,657.8	2,261.0
Fees and other revenues			1,985.8	2,030.5
Net investment income (loss)			1,987.3	1,776.1
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities			89.5	70.2
Net other-than-temporary impairment (losses) recoveries on fixed maturities, available-for-sale			(26.6)	(6.9)
Other-than-temporary impairment losses on fixed maturities, available-for-sale reclassified to (from) other comprehensive income			2.5	(12.0)
Net impairment (losses) recoveries on fixed maturities, available-for-sale			(24.1)	(18.9)
Net realized capital gains (losses)			65.4	51.3
Total revenues			7,696.3	6,118.9
Expenses
Benefits, claims and settlement expenses			4,641.2	3,110.3
Dividends to policyholders			60.0	60.7
Operating expenses			1,873.9	1,869.1
Total expenses			6,575.1	5,040.1
Income (loss) before income taxes			1,121.2	1,078.8
Income taxes (benefits)			166.2	154.3
Net income (loss)			955.0	924.5
Net income (loss) attributable to noncontrolling interest			28.0	8.3
Net income (loss) attributable to Principal Financial Group, Inc.			927.0	916.2
Net income (loss)			955.0	924.5
Other comprehensive income (loss)			2,149.5	(1,447.6)
Comprehensive income (loss)			3,104.5	(523.1)
Eliminations, Shelf Registration Debt Guarantor | Eliminations
Revenues
Fees and other revenues			(6.0)	(6.9)
Net investment income (loss)			4.0	6.4
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities				0.1
Net realized capital gains (losses)				0.1
Total revenues			(2.0)	(0.4)
Expenses
Operating expenses			(5.1)	(5.9)
Total expenses			(5.1)	(5.9)
Income (loss) before income taxes			3.1	5.5
Equity in the net income (loss) of subsidiaries			(1,810.9)	(1,820.9)
Net income (loss)			(1,807.8)	(1,815.4)
Net income (loss) attributable to Principal Financial Group, Inc.			(1,807.8)	(1,815.4)
Net income (loss)			(1,807.8)	(1,815.4)
Other comprehensive income (loss)			(4,258.6)	2,862.2
Comprehensive income (loss)			$ (6,066.4)	$ 1,046.8