Condensed Consolidating Financial Information - Shelf Registration Guarantor, Statements of Operations (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Revenues
Premiums and other considerations	$ 1,276.0	$ 1,682.4	$ 2,558.4	$ 2,598.8
Fees and other revenues	916.8	901.5	1,772.7	1,852.3
Net investment income (loss)	799.2	789.9	1,560.9	1,513.8
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities	43.7	(108.6)	228.4	(34.9)
Net other-than-temporary impairment (losses) recoveries on available-for-sale securities	(3.4)	(0.6)	(59.0)	13.4
Other-than-temporary impairment losses on fixed maturities, available-for-sale reclassified to (from) other comprehensive income	(6.6)	(5.4)	0.9	(26.9)
Net impairment (losses) recoveries on available-for-sale securities	(10.0)	(6.0)	(58.1)	(13.5)
Net realized capital gains (losses)	33.7	(114.6)	170.3	(48.4)
Total revenues	3,025.7	3,259.2	6,062.3	5,916.5
Expenses
Benefits, claims and settlement expenses	1,659.9	2,054.6	3,318.4	3,290.9
Dividends to policyholders	38.0	41.1	76.8	82.8
Operating expenses	927.9	869.0	1,827.4	1,790.2
Total expenses	2,625.8	2,964.7	5,222.6	5,163.9
Income (loss) before income taxes	399.9	294.5	839.7	752.6
Income taxes (benefits)	72.9	29.6	143.5	58.6
Net income (loss)	327.0	264.9	696.2	694.0
Net income (loss) attributable to noncontrolling interest	4.7	7.3	5.9	14.0
Net income (loss) attributable to Principal Financial Group, Inc.	322.3	257.6	690.3	680.0
Preferred stock dividends		8.3		16.5
Excess of redemption value over carrying value of preferred shares redeemed		8.2		8.2
Net income (loss) available to common stockholders	322.3	241.1	690.3	655.3
Net income (loss)	327.0	264.9	696.2	694.0
Other comprehensive income (loss)	339.1	(349.6)	878.3	(371.8)
Comprehensive income (loss)	$ 666.1	$ (84.7)	1,574.5	322.2
Principal Financial Group, Inc. Parent Only
Revenues
Net investment income (loss)			1.4	0.8
Total revenues			1.4	0.8
Expenses
Operating expenses			113.7	82.4
Total expenses			113.7	82.4
Income (loss) before income taxes			(112.3)	(81.6)
Income taxes (benefits)			(45.7)	(33.9)
Equity in the net income (loss) of subsidiaries			756.9	727.7
Net income (loss)			690.3	680.0
Net income (loss) attributable to Principal Financial Group, Inc.			690.3	680.0
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				655.3
Net income (loss)			690.3	680.0
Other comprehensive income (loss)			866.9	(436.8)
Comprehensive income (loss)			1,557.2	243.2
Principal Financial Services, Inc. Only
Revenues
Fees and other revenues			0.2	0.1
Net investment income (loss)			10.8
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities			(6.9)	4.5
Net realized capital gains (losses)			(6.9)	4.5
Total revenues			4.1	4.6
Expenses
Operating expenses			5.7	2.4
Total expenses			5.7	2.4
Income (loss) before income taxes			(1.6)	2.2
Income taxes (benefits)			(24.0)	2.0
Equity in the net income (loss) of subsidiaries			731.2	724.0
Net income (loss)			753.6	724.2
Net income (loss) attributable to Principal Financial Group, Inc.			753.6	724.2
Net income (loss) available to common stockholders				724.2
Net income (loss)			753.6	724.2
Other comprehensive income (loss)			842.6	(408.4)
Comprehensive income (loss)			1,596.2	315.8
Principal Life Insurance Company and Other Subsidiaries Combined
Revenues
Premiums and other considerations			2,558.4	2,598.8
Fees and other revenues			1,776.9	1,852.5
Net investment income (loss)			1,545.0	1,507.6
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities			235.4	(37.7)
Net other-than-temporary impairment (losses) recoveries on available-for-sale securities			(59.0)	13.4
Other-than-temporary impairment losses on fixed maturities, available-for-sale reclassified to (from) other comprehensive income			0.9	(26.9)
Net impairment (losses) recoveries on available-for-sale securities			(58.1)	(13.5)
Net realized capital gains (losses)			177.3	(51.2)
Total revenues			6,057.6	5,907.7
Expenses
Benefits, claims and settlement expenses			3,318.4	3,290.9
Dividends to policyholders			76.8	82.8
Operating expenses			1,712.1	1,705.5
Total expenses			5,107.3	5,079.2
Income (loss) before income taxes			950.3	828.5
Income taxes (benefits)			213.2	90.5
Net income (loss)			737.1	738.0
Net income (loss) attributable to noncontrolling interest			5.9	14.0
Net income (loss) attributable to Principal Financial Group, Inc.			731.2	724.0
Net income (loss) available to common stockholders				724.0
Net income (loss)			737.1	738.0
Other comprehensive income (loss)			875.1	(433.0)
Comprehensive income (loss)			1,612.2	305.0
Eliminations, Shelf Registration Debt Guarantor
Revenues
Fees and other revenues			(4.4)	(0.3)
Net investment income (loss)			3.7	5.4
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities			(0.1)	(1.7)
Net realized capital gains (losses)			(0.1)	(1.7)
Total revenues			(0.8)	3.4
Expenses
Operating expenses			(4.1)	(0.1)
Total expenses			(4.1)	(0.1)
Income (loss) before income taxes			3.3	3.5
Equity in the net income (loss) of subsidiaries			(1,488.1)	(1,451.7)
Net income (loss)			(1,484.8)	(1,448.2)
Net income (loss) attributable to Principal Financial Group, Inc.			(1,484.8)	(1,448.2)
Net income (loss) available to common stockholders				(1,448.2)
Net income (loss)			(1,484.8)	(1,448.2)
Other comprehensive income (loss)			(1,706.3)	906.4
Comprehensive income (loss)			$ (3,191.1)	$ (541.8)