Nature of Operations and Significant Accounting Policies (Details 2) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended						6 Months Ended	9 Months Ended	12 Months Ended		3 Months Ended				6 Months Ended	9 Months Ended	12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012 Adjustments Related to Deferred Acquisition Cost Accounting Change	Sep. 30, 2012 Adjustments Related to Deferred Acquisition Cost Accounting Change	Jun. 30, 2012 Adjustments Related to Deferred Acquisition Cost Accounting Change	Mar. 31, 2012 Adjustments Related to Deferred Acquisition Cost Accounting Change	Jun. 30, 2012 Adjustments Related to Deferred Acquisition Cost Accounting Change	Sep. 30, 2012 Adjustments Related to Deferred Acquisition Cost Accounting Change	Dec. 31, 2012 Adjustments Related to Deferred Acquisition Cost Accounting Change	Dec. 31, 2011 Adjustments Related to Deferred Acquisition Cost Accounting Change	Dec. 31, 2012 Adjustments Related to Deferred Acquisition Cost Accounting Change As originally reported	Sep. 30, 2012 Adjustments Related to Deferred Acquisition Cost Accounting Change As originally reported	Jun. 30, 2012 Adjustments Related to Deferred Acquisition Cost Accounting Change As originally reported	Mar. 31, 2012 Adjustments Related to Deferred Acquisition Cost Accounting Change As originally reported	Jun. 30, 2012 Adjustments Related to Deferred Acquisition Cost Accounting Change As originally reported	Sep. 30, 2012 Adjustments Related to Deferred Acquisition Cost Accounting Change As originally reported	Dec. 31, 2012 Adjustments Related to Deferred Acquisition Cost Accounting Change As originally reported	Dec. 31, 2011 Adjustments Related to Deferred Acquisition Cost Accounting Change As originally reported
Revenues
Premiums and other considerations	$ 694.7	$ 679.8	$ 700.1	$ 1,158.2	$ 681.3	$ 679.8	$ 1,361.1	$ 2,519.3	$ 3,219.4	$ 2,891.0	$ 700.1	$ 1,158.2	$ 681.3	$ 679.8	$ 1,361.1	$ 2,519.3	$ 3,219.4	$ 2,891.0
Fees and other revenues	733.6	598.0	717.6	675.0	636.1	598.0	1,234.1	1,909.1	2,626.7	2,526.7	717.6	675.0	636.1	598.0	1,234.1	1,909.1	2,626.7	2,526.7
Net investment income (loss)	789.3	824.8	845.3	783.8	801.0	824.8	1,625.8	2,409.6	3,254.9	3,375.3	845.3	783.8	801.0	824.8	1,625.8	2,409.6	3,254.9	3,375.3
Net realized capital gains (losses), excluding impairment losses on available-for-sale securities	(26.4)	22.1	56.3	122.1	32.2	22.1	54.3	176.4	232.7	75.0	56.3	122.1	32.2	22.1	54.3	176.4	232.7	75.0
Total other-than-temporary impairment losses on available-for-sale securities	(44.7)	(33.7)	(9.5)	(43.6)	(49.1)	(33.7)	(82.8)	(126.4)	(135.9)	(147.6)	(9.5)	(43.6)	(49.1)	(33.7)	(82.8)	(126.4)	(135.9)	(147.6)
Other-than-temporary impairment losses on fixed maturities, available-for-sale reclassified to (from) other comprehensive income	20.2	4.9	(13.9)	9.2	17.1	4.9	22.0	31.2	17.3	(49.7)	(13.9)	9.2	17.1	4.9	22.0	31.2	17.3	(49.7)
Net impairment losses on available-for-sale securities	(24.5)	(28.8)	(23.4)	(34.4)	(32.0)	(28.8)	(60.8)	(95.2)	(118.6)	(197.3)	(23.4)	(34.4)	(32.0)	(28.8)	(60.8)	(95.2)	(118.6)	(197.3)
Net realized capital gains (losses)	(50.9)	(6.7)	32.9	87.7	0.2	(6.7)	(6.5)	81.2	114.1	(122.3)	32.9	87.7	0.2	(6.7)	(6.5)	81.2	114.1	(122.3)
Total revenues	2,166.7	2,095.9	2,295.9	2,704.7	2,118.6	2,095.9	4,214.5	6,919.2	9,215.1	8,670.7	2,295.9	2,704.7	2,118.6	2,095.9	4,214.5	6,919.2	9,215.1	8,670.7
Expenses
Benefits, claims and settlement expenses	1,094.5	1,212.5	1,154.4	1,647.0	1,110.0	1,212.5	2,322.5	3,969.5	5,123.9	4,616.6	1,154.4	1,647.0	1,110.0	1,212.5	2,322.5	3,969.5	5,123.9	4,616.6
Dividends to policyholders	48.3	50.3	48.2	49.7	49.5	50.3	99.8	149.5	197.7	210.2	48.2	49.7	49.5	50.3	99.8	149.5	197.7	210.2
Operating expenses	795.7	555.1	832.4	816.4	729.6	555.1	1,284.7	2,101.1	2,933.5	2,971.1	827.4	826.6	724.1	556.0	1,280.1	2,106.7	2,934.1	2,950.8
Total expenses	1,938.5	1,817.9	2,035.0	2,513.1	1,889.1	1,817.9	3,707.0	6,220.1	8,255.1	7,797.9	2,030.0	2,523.3	1,883.6	1,818.8	3,702.4	6,225.7	8,255.7	7,777.6
Income (loss) before income taxes	228.2	278.0	260.9	191.6	229.5	278.0	507.5	699.1	960.0	872.8	265.9	181.4	235.0	277.1	512.1	693.5	959.4	893.1
Income taxes (benefits)	38.2	56.7	34.2	(7.2)	50.9	56.7	107.6	100.4	134.6	198.3	35.5	(9.9)	50.9	58.2	109.1	99.2	134.7	204.2
Net income (loss)	190.0	221.3	226.7	198.8	178.6	221.3	399.9	598.7	825.4	674.5	230.4	191.3	184.1	218.9	403.0	594.3	824.7	688.9
Net income (loss) attributable to noncontrolling interest	3.5	9.2	3.5	3.4	2.7	9.2	11.9	15.3	18.8	36.2	3.5	3.4	2.7	9.2	11.9	15.3	18.8	36.2
Net income (loss) attributable to Principal Financial Group, Inc.	186.5	212.1	223.2	195.4	175.9	212.1	388.0	583.4	806.6	638.3	226.9	187.9	181.4	209.7	391.1	579.0	805.9	652.7
Preferred stock dividends	8.2	8.2	8.3	8.2	8.3	8.2	16.5	24.7	33.0	33.0	8.3	8.2	8.3	8.2	16.5	24.7	33.0	33.0
Net income (loss) available to common stockholders	$ 178.3	$ 203.9	$ 214.9	$ 187.2	$ 167.6	$ 203.9	$ 371.5	$ 558.7	$ 773.6	$ 605.3	$ 218.6	$ 179.7	$ 173.1	$ 201.5	$ 374.6	$ 554.3	$ 772.9	$ 619.7
Earnings per common share
Basic earnings per common share (in dollars per share)	$ 0.61	$ 0.68	$ 0.72	$ 0.64	$ 0.56	$ 0.68	$ 1.24	$ 1.87	$ 2.60	$ 1.92	$ 0.74	$ 0.61	$ 0.58	$ 0.67	$ 1.25	$ 1.86	$ 2.60	$ 1.97
Diluted earnings per common share (in dollars per share)	$ 0.61	$ 0.68	$ 0.72	$ 0.63	$ 0.56	$ 0.68	$ 1.22	$ 1.85	$ 2.58	$ 1.91	$ 0.74	$ 0.60	$ 0.58	$ 0.66	$ 1.24	$ 1.84	$ 2.57	$ 1.95