Condensed Consolidating Financial Information - Guarantor, Statements of Operations (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenues
Premiums and other considerations	$ 1,265.8	$ 1,452.7	$ 2,261.0	$ 2,700.7
Fees and other revenues	1,012.6	955.1	2,023.9	1,895.7
Net investment income (loss)	889.1	866.4	1,791.3	1,743.8
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities	76.4	(81.7)	61.2	(69.5)
Net other-than-temporary impairment (losses) recoveries on available-for-sale securities	(8.2)	(2.4)	(6.9)	(29.7)
Other-than-temporary impairment losses on fixed maturities, available-for-sale reclassified to (from) other comprehensive income	(0.8)	(11.8)	(12.0)	(13.3)
Net impairment (losses) recoveries on available-for-sale securities	(9.0)	(14.2)	(18.9)	(43.0)
Net realized capital gains (losses)	67.4	(95.9)	42.3	(112.5)
Total revenues	3,234.9	3,178.3	6,118.5	6,227.7
Expenses
Benefits, claims and settlement expenses	1,699.2	1,858.8	3,110.3	3,516.1
Dividends to policyholders	30.2	25.2	60.7	60.1
Operating expenses	990.1	928.7	1,975.1	1,871.9
Total expenses	2,719.5	2,812.7	5,146.1	5,448.1
Income (loss) before income taxes	515.4	365.6	972.4	779.6
Income taxes (benefits)	55.9	50.8	110.4	111.2
Net income (loss)	459.5	314.8	862.0	668.4
Net income (loss) attributable to noncontrolling interest	2.9	5.3	8.3	10.0
Net income (loss) attributable to Principal Financial Group, Inc.	456.6	309.5	853.7	658.4
Net income (loss)	459.5	314.8	862.0	668.4
Other comprehensive income (loss)	(672.3)	338.7	(1,408.4)	559.8
Comprehensive income (loss)	$ (212.8)	$ 653.5	(546.4)	1,228.2
Principal Financial Group, Inc. Parent Only
Revenues
Fees and other revenues				7.8
Net investment income (loss)			11.8	4.3
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities			(8.6)
Net realized capital gains (losses)			(8.6)
Total revenues			3.2	12.1
Expenses
Operating expenses			95.2	102.0
Total expenses			95.2	102.0
Income (loss) before income taxes			(92.0)	(89.9)
Income taxes (benefits)			(41.0)	(36.3)
Equity in the net income (loss) of subsidiaries			904.7	712.0
Net income (loss)			853.7	658.4
Net income (loss) attributable to Principal Financial Group, Inc.			853.7	658.4
Net income (loss)			853.7	658.4
Other comprehensive income (loss)			(1,398.2)	557.0
Comprehensive income (loss)			(544.5)	1,215.4
Principal Life Insurance Company Only
Revenues
Premiums and other considerations			1,941.7	2,485.7
Fees and other revenues			1,091.3	1,038.0
Net investment income (loss)			1,282.0	1,211.4
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities			(98.6)	(14.5)
Net other-than-temporary impairment (losses) recoveries on available-for-sale securities			(4.6)	(29.5)
Other-than-temporary impairment losses on fixed maturities, available-for-sale reclassified to (from) other comprehensive income			(11.4)	(11.4)
Net impairment (losses) recoveries on available-for-sale securities			(16.0)	(40.9)
Net realized capital gains (losses)			(114.6)	(55.4)
Total revenues			4,200.4	4,679.7
Expenses
Benefits, claims and settlement expenses			2,640.6	3,159.1
Dividends to policyholders			60.7	60.1
Operating expenses			1,144.3	1,035.2
Total expenses			3,845.6	4,254.4
Income (loss) before income taxes			354.8	425.3
Income taxes (benefits)			7.2	44.4
Equity in the net income (loss) of subsidiaries			313.2	92.8
Net income (loss)			660.8	473.7
Net income (loss) attributable to Principal Financial Group, Inc.			660.8	473.7
Net income (loss)			660.8	473.7
Other comprehensive income (loss)			(1,219.2)	434.1
Comprehensive income (loss)			(558.4)	907.8
Income (loss) from discontinued operations, net of related income taxes				35.5
Principal Financial Services, Inc. and Other Subsidiaries Combined
Revenues
Premiums and other considerations			319.3	215.0
Fees and other revenues			1,126.7	1,058.7
Net investment income (loss)			1,152.8	963.3
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities			168.4	(54.3)
Net other-than-temporary impairment (losses) recoveries on available-for-sale securities			(2.3)	(0.2)
Other-than-temporary impairment losses on fixed maturities, available-for-sale reclassified to (from) other comprehensive income			(0.6)	(1.9)
Net impairment (losses) recoveries on available-for-sale securities			(2.9)	(2.1)
Net realized capital gains (losses)			165.5	(56.4)
Total revenues			2,764.3	2,180.6
Expenses
Benefits, claims and settlement expenses			475.3	362.3
Operating expenses			916.7	910.9
Total expenses			1,392.0	1,273.2
Income (loss) before income taxes			1,372.3	907.4
Income taxes (benefits)			144.5	106.0
Equity in the net income (loss) of subsidiaries			(320.3)	(82.4)
Net income (loss)			907.5	719.0
Net income (loss) attributable to noncontrolling interest			8.3	10.0
Net income (loss) attributable to Principal Financial Group, Inc.			899.2	709.0
Net income (loss)			907.5	719.0
Other comprehensive income (loss)			(1,500.5)	588.2
Comprehensive income (loss)			(593.0)	1,307.2
Eliminations, Notes Guarantor
Revenues
Fees and other revenues			(194.1)	(208.8)
Net investment income (loss)			(655.3)	(435.2)
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities				(0.7)
Net realized capital gains (losses)				(0.7)
Total revenues			(849.4)	(644.7)
Expenses
Benefits, claims and settlement expenses			(5.6)	(5.3)
Operating expenses			(181.1)	(176.2)
Total expenses			(186.7)	(181.5)
Income (loss) before income taxes			(662.7)	(463.2)
Income taxes (benefits)			(0.3)	(2.9)
Equity in the net income (loss) of subsidiaries			(897.6)	(722.4)
Net income (loss)			(1,560.0)	(1,182.7)
Net income (loss) attributable to Principal Financial Group, Inc.			(1,560.0)	(1,182.7)
Net income (loss)			(1,560.0)	(1,182.7)
Other comprehensive income (loss)			2,709.5	(1,019.5)
Comprehensive income (loss)			$ 1,149.5	$ (2,202.2)