Condensed Consolidating Financial Information - Shelf Registration Guarantor, Statements of Operations (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Revenues
Premiums and other considerations	$ 1,248.0	$ 1,282.4
Fees and other revenues	940.6	855.9
Net investment income (loss)	877.4	761.7
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities	12.2	184.7
Net other-than-temporary impairment (losses) recoveries on available-for-sale securities	(27.3)	(55.6)
Other-than-temporary impairment losses on fixed maturities, available-for-sale reclassified to (from) other comprehensive income	(1.5)	7.5
Net impairment (losses) recoveries on available-for-sale securities	(28.8)	(48.1)
Net realized capital gains (losses)	(16.6)	136.6
Total revenues	3,049.4	3,036.6
Expenses
Benefits, claims and settlement expenses	1,657.3	1,658.5
Dividends to policyholders	34.9	38.8
Operating expenses	943.2	899.5
Total expenses	2,635.4	2,596.8
Income (loss) before income taxes	414.0	439.8
Income taxes (benefits)	60.4	70.6
Net income (loss)	353.6	369.2
Net income (loss) attributable to noncontrolling interest	4.7	1.2
Net income (loss) attributable to Principal Financial Group, Inc.	348.9	368.0
Net income (loss)	353.6	369.2
Other comprehensive income (loss)	221.1	539.2
Comprehensive income (loss)	574.7	908.4
Principal Financial Group, Inc. Parent Only
Revenues
Fees and other revenues	7.8
Net investment income (loss)	1.8	0.7
Total revenues	9.6	0.7
Expenses
Operating expenses	50.8	56.8
Total expenses	50.8	56.8
Income (loss) before income taxes	(41.2)	(56.1)
Income taxes (benefits)	(16.5)	(22.8)
Equity in the net income (loss) of subsidiaries	373.6	401.3
Net income (loss)	348.9	368.0
Net income (loss) attributable to Principal Financial Group, Inc.	348.9	368.0
Net income (loss)	348.9	368.0
Other comprehensive income (loss)	217.7	532.0
Comprehensive income (loss)	566.6	900.0
Principal Financial Services, Inc. Only
Revenues
Fees and other revenues	0.1	0.1
Net investment income (loss)	(0.4)	6.6
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities	2.3	(6.0)
Net realized capital gains (losses)	2.3	(6.0)
Total revenues	2.0	0.7
Expenses
Operating expenses	22.9	3.8
Total expenses	22.9	3.8
Income (loss) before income taxes	(20.9)	(3.1)
Income taxes (benefits)	(9.3)	(20.4)
Equity in the net income (loss) of subsidiaries	383.6	382.3
Net income (loss)	372.0	399.6
Net income (loss) attributable to Principal Financial Group, Inc.	372.0	399.6
Net income (loss)	372.0	399.6
Other comprehensive income (loss)	206.5	519.8
Comprehensive income (loss)	578.5	919.4
Principal Life Insurance Company and Other Subsidiaries Combined
Revenues
Premiums and other considerations	1,248.0	1,282.4
Fees and other revenues	934.0	859.1
Net investment income (loss)	874.1	752.8
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities	9.9	190.5
Net other-than-temporary impairment (losses) recoveries on available-for-sale securities	(27.3)	(55.6)
Other-than-temporary impairment losses on fixed maturities, available-for-sale reclassified to (from) other comprehensive income	(1.5)	7.5
Net impairment (losses) recoveries on available-for-sale securities	(28.8)	(48.1)
Net realized capital gains (losses)	(18.9)	142.4
Total revenues	3,037.2	3,036.7
Expenses
Benefits, claims and settlement expenses	1,657.3	1,658.5
Dividends to policyholders	34.9	38.8
Operating expenses	870.5	842.1
Total expenses	2,562.7	2,539.4
Income (loss) before income taxes	474.5	497.3
Income taxes (benefits)	86.2	113.8
Net income (loss)	388.3	383.5
Net income (loss) attributable to noncontrolling interest	4.7	1.2
Net income (loss) attributable to Principal Financial Group, Inc.	383.6	382.3
Net income (loss)	388.3	383.5
Other comprehensive income (loss)	212.7	537.1
Comprehensive income (loss)	601.0	920.6
Eliminations, Shelf Registration Debt Guarantor
Revenues
Fees and other revenues	(1.3)	(3.3)
Net investment income (loss)	1.9	1.6
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities		0.2
Net realized capital gains (losses)		0.2
Total revenues	0.6	(1.5)
Expenses
Operating expenses	(1.0)	(3.2)
Total expenses	(1.0)	(3.2)
Income (loss) before income taxes	1.6	1.7
Equity in the net income (loss) of subsidiaries	(757.2)	(783.6)
Net income (loss)	(755.6)	(781.9)
Net income (loss) attributable to Principal Financial Group, Inc.	(755.6)	(781.9)
Net income (loss)	(755.6)	(781.9)
Other comprehensive income (loss)	(415.8)	(1,049.7)
Comprehensive income (loss)	$ (1,171.4)	$ (1,831.6)