Condensed Consolidating Financial Information - Shelf Registration Guarantor, Statements of Operations (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Revenues
Premiums and other considerations	$ 1,282.4	$ 916.4
Fees and other revenues	855.9	950.8
Net investment income (loss)	761.7	723.9
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities	184.7	73.7
Net other-than-temporary impairment (losses) recoveries on available-for-sale securities	(55.6)	14.0
Other-than-temporary impairment losses on fixed maturities, available-for-sale reclassified to (from) other comprehensive income	7.5	(21.5)
Net impairment (losses) recoveries on available-for-sale securities	(48.1)	(7.5)
Net realized capital gains (losses)	136.6	66.2
Total revenues	3,036.6	2,657.3
Expenses
Benefits, claims and settlement expenses	1,658.5	1,236.3
Dividends to policyholders	38.8	41.7
Operating expenses	899.5	921.2
Total expenses	2,596.8	2,199.2
Income (loss) before income taxes	439.8	458.1
Income taxes (benefits)	70.6	29.0
Net income (loss)	369.2	429.1
Net income (loss) attributable to noncontrolling interest	1.2	6.7
Net income (loss) attributable to Principal Financial Group, Inc.	368.0	422.4
Preferred stock dividends		8.2
Net income (loss) available to common stockholders	368.0	414.2
Net income (loss)	369.2	429.1
Other comprehensive income (loss)	539.2	(22.2)
Comprehensive income (loss)	908.4	406.9
Principal Financial Group, Inc. Parent Only
Revenues
Net investment income (loss)	0.7	0.3
Total revenues	0.7	0.3
Expenses
Operating expenses	56.8	38.6
Total expenses	56.8	38.6
Income (loss) before income taxes	(56.1)	(38.3)
Income taxes (benefits)	(22.8)	(16.3)
Equity in the net income (loss) of subsidiaries	401.3	444.4
Net income (loss)	368.0	422.4
Net income (loss) attributable to Principal Financial Group, Inc.	368.0	422.4
Preferred stock dividends		8.2
Net income (loss) available to common stockholders		414.2
Net income (loss)	368.0	422.4
Other comprehensive income (loss)	532.0	(85.7)
Comprehensive income (loss)	900.0	336.7
Principal Financial Services, Inc. Only
Revenues
Fees and other revenues	0.1	0.1
Net investment income (loss)	6.6	0.2
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities	(6.0)	2.1
Net realized capital gains (losses)	(6.0)	2.1
Total revenues	0.7	2.4
Expenses
Operating expenses	3.8	(0.7)
Total expenses	3.8	(0.7)
Income (loss) before income taxes	(3.1)	3.1
Income taxes (benefits)	(20.4)	4.3
Equity in the net income (loss) of subsidiaries	382.3	443.8
Net income (loss)	399.6	442.6
Net income (loss) attributable to Principal Financial Group, Inc.	399.6	442.6
Net income (loss) available to common stockholders		442.6
Net income (loss)	399.6	442.6
Other comprehensive income (loss)	519.8	(41.0)
Comprehensive income (loss)	919.4	401.6
Principal Life Insurance Company and Other Subsidiaries Combined
Revenues
Premiums and other considerations	1,282.4	916.4
Fees and other revenues	859.1	950.8
Net investment income (loss)	752.8	721.8
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities	190.5	71.5
Net other-than-temporary impairment (losses) recoveries on available-for-sale securities	(55.6)	14.0
Other-than-temporary impairment losses on fixed maturities, available-for-sale reclassified to (from) other comprehensive income	7.5	(21.5)
Net impairment (losses) recoveries on available-for-sale securities	(48.1)	(7.5)
Net realized capital gains (losses)	142.4	64.0
Total revenues	3,036.7	2,653.0
Expenses
Benefits, claims and settlement expenses	1,658.5	1,236.3
Dividends to policyholders	38.8	41.7
Operating expenses	842.1	883.5
Total expenses	2,539.4	2,161.5
Income (loss) before income taxes	497.3	491.5
Income taxes (benefits)	113.8	41.0
Net income (loss)	383.5	450.5
Net income (loss) attributable to noncontrolling interest	1.2	6.7
Net income (loss) attributable to Principal Financial Group, Inc.	382.3	443.8
Net income (loss) available to common stockholders		443.8
Net income (loss)	383.5	450.5
Other comprehensive income (loss)	537.1	(71.3)
Comprehensive income (loss)	920.6	379.2
Eliminations, Shelf Registration Debt Guarantor
Revenues
Fees and other revenues	(3.3)	(0.1)
Net investment income (loss)	1.6	1.6
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities	0.2	0.1
Net realized capital gains (losses)	0.2	0.1
Total revenues	(1.5)	1.6
Expenses
Operating expenses	(3.2)	(0.2)
Total expenses	(3.2)	(0.2)
Income (loss) before income taxes	1.7	1.8
Equity in the net income (loss) of subsidiaries	(783.6)	(888.2)
Net income (loss)	(781.9)	(886.4)
Net income (loss) attributable to Principal Financial Group, Inc.	(781.9)	(886.4)
Net income (loss) available to common stockholders		(886.4)
Net income (loss)	(781.9)	(886.4)
Other comprehensive income (loss)	(1,049.7)	175.8
Comprehensive income (loss)	$ (1,831.6)	$ (710.6)