Schedule II - Condensed Financial Information of Registrant (Parent Only) - 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
Fees and other revenues									$ 3,892.3	$ 3,627.4	$ 3,653.1
Net investment income									3,459.3	3,296.5	3,052.1
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
Other operating costs and 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) 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
Principal Financial Group, Inc. Parent Only
Revenues
Fees and other revenues									7.8
Net investment income									9.7	3.4	1.6
Net realized capital gains (losses)									(0.6)
Total revenues									16.9	3.4	1.6
Expenses
Other operating costs and 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) 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									$ 2,310.4	$ 1,316.5	$ 1,209.3