Condensed Consolidating Financial Information - Shelf Registration Guarantor, Statements of Operations (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Revenues
Premiums and other considerations	$ 1,452.7	$ 1,276.0	$ 2,700.7	$ 2,558.4
Fees and other revenues	955.1	916.8	1,895.7	1,772.7
Net investment income (loss)	866.4	799.2	1,743.8	1,560.9
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities	(81.7)	43.7	(69.5)	228.4
Net other-than-temporary impairment (losses) recoveries on available-for-sale securities	(2.4)	(3.4)	(29.7)	(59.0)
Other-than-temporary impairment losses on fixed maturities, available-for-sale reclassified to (from) other comprehensive income	(11.8)	(6.6)	(13.3)	0.9
Net impairment (losses) recoveries on available-for-sale securities	(14.2)	(10.0)	(43.0)	(58.1)
Net realized capital gains (losses)	(95.9)	33.7	(112.5)	170.3
Total revenues	3,178.3	3,025.7	6,227.7	6,062.3
Expenses
Benefits, claims and settlement expenses	1,858.8	1,659.9	3,516.1	3,318.4
Dividends to policyholders	25.2	38.0	60.1	76.8
Operating expenses	928.7	927.9	1,871.9	1,827.4
Total expenses	2,812.7	2,625.8	5,448.1	5,222.6
Income (loss) before income taxes	365.6	399.9	779.6	839.7
Income taxes (benefits)	50.8	72.9	111.2	143.5
Net income (loss)	314.8	327.0	668.4	696.2
Net income (loss) attributable to noncontrolling interest	5.3	4.7	10.0	5.9
Net income (loss) attributable to Principal Financial Group, Inc.	309.5	322.3	658.4	690.3
Net income (loss)	314.8	327.0	668.4	696.2
Other comprehensive income (loss)	338.7	339.1	559.8	878.3
Comprehensive income (loss)	$ 653.5	$ 666.1	1,228.2	1,574.5
Principal Financial Group, Inc. Parent Only
Revenues
Fees and other revenues			7.8
Net investment income (loss)			4.3	1.4
Total revenues			12.1	1.4
Expenses
Operating expenses			102.0	113.7
Total expenses			102.0	113.7
Income (loss) before income taxes			(89.9)	(112.3)
Income taxes (benefits)			(36.3)	(45.7)
Equity in the net income (loss) of subsidiaries			712.0	756.9
Net income (loss)			658.4	690.3
Net income (loss) attributable to Principal Financial Group, Inc.			658.4	690.3
Net income (loss)			658.4	690.3
Other comprehensive income (loss)			557.0	866.9
Comprehensive income (loss)			1,215.4	1,557.2
Principal Financial Services, Inc. Only
Revenues
Fees and other revenues			0.1	0.2
Net investment income (loss)			(1.1)	10.8
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities			5.0	(6.9)
Net realized capital gains (losses)			5.0	(6.9)
Total revenues			4.0	4.1
Expenses
Operating expenses			29.8	5.7
Total expenses			29.8	5.7
Income (loss) before income taxes			(25.8)	(1.6)
Income taxes (benefits)			(11.7)	(24.0)
Equity in the net income (loss) of subsidiaries			723.1	731.2
Net income (loss)			709.0	753.6
Net income (loss) attributable to Principal Financial Group, Inc.			709.0	753.6
Net income (loss)			709.0	753.6
Other comprehensive income (loss)			534.5	842.6
Comprehensive income (loss)			1,243.5	1,596.2
Principal Life Insurance Company and Other Subsidiaries Combined
Revenues
Premiums and other considerations			2,700.7	2,558.4
Fees and other revenues			1,891.6	1,776.9
Net investment income (loss)			1,738.0	1,545.0
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities			(74.5)	235.4
Net other-than-temporary impairment (losses) recoveries on available-for-sale securities			(29.7)	(59.0)
Other-than-temporary impairment losses on fixed maturities, available-for-sale reclassified to (from) other comprehensive income			(13.3)	0.9
Net impairment (losses) recoveries on available-for-sale securities			(43.0)	(58.1)
Net realized capital gains (losses)			(117.5)	177.3
Total revenues			6,212.8	6,057.6
Expenses
Benefits, claims and settlement expenses			3,516.1	3,318.4
Dividends to policyholders			60.1	76.8
Operating expenses			1,743.4	1,712.1
Total expenses			5,319.6	5,107.3
Income (loss) before income taxes			893.2	950.3
Income taxes (benefits)			160.1	213.2
Net income (loss)			733.1	737.1
Net income (loss) attributable to noncontrolling interest			10.0	5.9
Net income (loss) attributable to Principal Financial Group, Inc.			723.1	731.2
Net income (loss)			733.1	737.1
Other comprehensive income (loss)			532.0	875.1
Comprehensive income (loss)			1,265.1	1,612.2
Eliminations, Shelf Registration Debt Guarantor
Revenues
Fees and other revenues			(3.8)	(4.4)
Net investment income (loss)			2.6	3.7
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities				(0.1)
Net realized capital gains (losses)				(0.1)
Total revenues			(1.2)	(0.8)
Expenses
Operating expenses			(3.3)	(4.1)
Total expenses			(3.3)	(4.1)
Income (loss) before income taxes			2.1	3.3
Income taxes (benefits)			(0.9)
Equity in the net income (loss) of subsidiaries			(1,435.1)	(1,488.1)
Net income (loss)			(1,432.1)	(1,484.8)
Net income (loss) attributable to Principal Financial Group, Inc.			(1,432.1)	(1,484.8)
Net income (loss)			(1,432.1)	(1,484.8)
Other comprehensive income (loss)			(1,063.7)	(1,706.3)
Comprehensive income (loss)			$ (2,495.8)	$ (3,191.1)