Condensed Consolidating Financial Information - Guarantor, Statements of Operations (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues
Premiums and other considerations									$ 6,217.4	$ 5,299.1	$ 5,310.3
Fees and other revenues									3,892.3	3,627.4	3,653.1
Net investment income (loss)									3,459.3	3,296.5	3,052.1
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities									606.0	269.5	(20.9)
Net other-than-temporary impairment (losses) recoveries on available-for-sale securities									(28.7)	(98.8)	(0.8)
Other-than-temporary impairment losses on fixed maturities, available-for-sale reclassified to (from) other comprehensive income									(53.1)	0.4	(29.4)
Net impairment (losses) recoveries on available-for-sale securities									(81.8)	(98.4)	(30.2)
Net realized capital gains (losses)									524.2	171.1	(51.1)
Total revenues	$ 3,237.7	$ 4,627.8	$ 3,178.3	$ 3,049.4	$ 3,513.8	$ 2,818.0	$ 3,025.7	$ 3,036.6	14,093.2	12,394.1	11,964.4
Expenses
Benefits, claims and settlement expenses									7,822.6	6,913.2	6,697.7
Dividends to policyholders									124.6	156.6	163.5
Operating expenses									3,893.8	3,732.6	3,672.4
Total expenses	2,922.9	3,470.0	2,812.7	2,635.4	3,132.5	2,447.3	2,625.8	2,596.8	11,841.0	10,802.4	10,533.6
Income (loss) before income taxes									2,252.2	1,591.7	1,430.8
Income taxes (benefits)									(72.3)	229.9	177.6
Net income (loss)	842.9	813.2	314.8	353.6	338.2	327.4	327.0	369.2	2,324.5	1,361.8	1,253.2
Net income (loss) attributable to noncontrolling interest									14.1	45.3	19.2
Net income (loss) attributable to Principal Financial Group, Inc.									2,310.4	1,316.5	1,234.0
Preferred stock dividends											16.5
Excess of redemption value over carrying value of preferred shares redeemed											8.2
Net income (loss) available to common stockholders	841.8	810.2	309.5	348.9	318.0	308.2	322.3	368.0	2,310.4	1,316.5	1,209.3
Net income (loss)	$ 842.9	$ 813.2	$ 314.8	$ 353.6	$ 338.2	$ 327.4	$ 327.0	$ 369.2	2,324.5	1,361.8	1,253.2
Other comprehensive income (loss)									844.3	221.0	(943.1)
Comprehensive income (loss)									3,168.8	1,582.8	310.1
Principal Financial Group, Inc. Parent Only
Revenues
Fees and other revenues									7.8
Net investment income (loss)									9.7	3.4	1.6
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities									(0.6)
Net realized capital gains (losses)									(0.6)
Total revenues									16.9	3.4	1.6
Expenses
Operating expenses									238.1	312.3	175.4
Total expenses									238.1	312.3	175.4
Income (loss) before income taxes									(221.2)	(308.9)	(173.8)
Income taxes (benefits)									63.1	(134.9)	(71.2)
Equity in the net income (loss) of subsidiaries									2,594.7	1,490.5	1,336.6
Net income (loss)									2,310.4	1,316.5	1,234.0
Net income (loss) attributable to Principal Financial Group, Inc.									2,310.4	1,316.5	1,234.0
Preferred stock dividends											16.5
Excess of redemption value over carrying value of preferred shares redeemed											8.2
Net income (loss) available to common stockholders											1,209.3
Net income (loss)									2,310.4	1,316.5	1,234.0
Other comprehensive income (loss)									840.3	218.8	(988.0)
Comprehensive income (loss)									3,150.7	1,535.3	246.0
Principal Life Insurance Company Only
Revenues
Premiums and other considerations									5,777.7	4,869.0	4,950.0
Fees and other revenues									2,140.9	1,956.1	2,014.1
Net investment income (loss)									2,533.1	2,300.2	2,164.0
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities									(84.5)	210.4	357.2
Net other-than-temporary impairment (losses) recoveries on available-for-sale securities									(28.2)	(92.2)	(0.4)
Other-than-temporary impairment losses on fixed maturities, available-for-sale reclassified to (from) other comprehensive income									(46.4)	(3.0)	(29.4)
Net impairment (losses) recoveries on available-for-sale securities									(74.6)	(95.2)	(29.8)
Net realized capital gains (losses)									(159.1)	115.2	327.4
Total revenues									10,292.6	9,240.5	9,455.5
Expenses
Benefits, claims and settlement expenses									7,153.5	6,177.9	6,047.1
Dividends to policyholders									124.6	156.6	163.5
Operating expenses									2,185.4	2,113.9	2,189.2
Total expenses									9,463.5	8,448.4	8,399.8
Income (loss) before income taxes									829.1	792.1	1,055.7
Income taxes (benefits)									(540.1)	97.9	235.7
Equity in the net income (loss) of subsidiaries									901.5	395.9	114.6
Net income (loss)									2,270.7	1,090.1	934.6
Net income (loss) attributable to Principal Financial Group, Inc.									2,270.7	1,090.1	934.6
Net income (loss) available to common stockholders											934.6
Net income (loss)									2,270.7	1,090.1	934.6
Other comprehensive income (loss)									611.6	116.5	(446.9)
Comprehensive income (loss)									2,882.3	1,206.6	487.7
Income (loss) from discontinued operations, net of related income taxes									35.5	132.6	120.6
Principal Financial Services, Inc. and Other Subsidiaries Combined
Revenues
Premiums and other considerations									439.7	430.1	360.3
Fees and other revenues									2,163.4	2,061.1	2,037.1
Net investment income (loss)									3,180.9	2,004.7	1,632.5
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities									691.8	48.1	(378.1)
Net other-than-temporary impairment (losses) recoveries on available-for-sale securities									(0.5)	(6.6)	(0.4)
Other-than-temporary impairment losses on fixed maturities, available-for-sale reclassified to (from) other comprehensive income									(6.7)	3.4
Net impairment (losses) recoveries on available-for-sale securities									(7.2)	(3.2)	(0.4)
Net realized capital gains (losses)									684.6	44.9	(378.5)
Total revenues									6,468.6	4,540.8	3,651.4
Expenses
Benefits, claims and settlement expenses									679.9	746.3	661.9
Operating expenses									1,823.2	1,629.7	1,643.3
Total expenses									2,503.1	2,376.0	2,305.2
Income (loss) before income taxes									3,965.5	2,164.8	1,346.2
Income taxes (benefits)									376.4	270.0	14.7
Equity in the net income (loss) of subsidiaries									(1,005.3)	(366.0)	17.4
Net income (loss)									2,583.8	1,528.8	1,348.9
Net income (loss) attributable to noncontrolling interest									14.1	45.3	19.2
Net income (loss) attributable to Principal Financial Group, Inc.									2,569.7	1,483.5	1,329.7
Net income (loss) available to common stockholders											1,329.7
Net income (loss)									2,583.8	1,528.8	1,348.9
Other comprehensive income (loss)									877.6	218.1	(935.0)
Comprehensive income (loss)									3,461.4	1,746.9	413.9
Eliminations, Notes Guarantor
Revenues
Fees and other revenues									(419.8)	(389.8)	(398.1)
Net investment income (loss)									(2,264.4)	(1,011.8)	(746.0)
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities									(0.7)	11.0
Net realized capital gains (losses)									(0.7)	11.0
Total revenues									(2,684.9)	(1,390.6)	(1,144.1)
Expenses
Benefits, claims and settlement expenses									(10.8)	(11.0)	(11.3)
Operating expenses									(352.9)	(323.3)	(335.5)
Total expenses									(363.7)	(334.3)	(346.8)
Income (loss) before income taxes									(2,321.2)	(1,056.3)	(797.3)
Income taxes (benefits)									28.3	(3.1)	(1.6)
Equity in the net income (loss) of subsidiaries									(2,490.9)	(1,520.4)	(1,468.6)
Net income (loss)									(4,840.4)	(2,573.6)	(2,264.3)
Net income (loss) attributable to Principal Financial Group, Inc.									(4,840.4)	(2,573.6)	(2,264.3)
Net income (loss) available to common stockholders											(2,264.3)
Net income (loss)									(4,840.4)	(2,573.6)	(2,264.3)
Other comprehensive income (loss)									(1,485.2)	(332.4)	1,426.8
Comprehensive income (loss)									$ (6,325.6)	$ (2,906.0)	$ (837.5)