Nature of Operations and Significant Accounting Policies (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011
Assets
Fixed maturities, available-for-sale	$ 51,084.1	$ 50,939.3
Fixed maturities, trading	628.9	626.7
Equity securities, available-for-sale	147.0	136.5
Equity securities, trading	732.2	252.8
Mortgage loans	11,577.9	11,519.7
Real estate	1,202.0	1,180.3
Policy loans	866.5	864.9
Other investments	3,187.2	3,291.1
Total investments	69,425.8	68,811.3
Cash and cash equivalents	1,661.2	4,177.2	1,651.9	2,833.9
Accrued investment income	584.5	584.4
Premiums due and other receivables	1,175.2	1,084.4
Deferred acquisition costs	2,715.5	2,590.0
Property and equipment	472.6	464.2
Goodwill	1,197.4	543.4		482.3
Other intangibles	1,590.3	914.7
Separate account assets	120,906.1	81,653.8
Other assets	1,034.3	1,006.8
Total assets	200,762.9	161,830.2
Liabilities
Contractholder funds	36,701.6	37,786.5
Future policy benefits and claims	22,689.5	22,436.2
Other policyholder funds	811.2	716.4
Short-term debt	46.5	40.8
Long-term debt	2,790.5	2,671.3
Income taxes currently payable	9.8	15.3
Deferred income taxes	791.4	600.0
Separate account liabilities	120,906.1	81,653.8
Other liabilities	6,130.8	6,146.1
Total liabilities	190,877.4	152,066.4
Redeemable noncontrolling interest	58.9	60.4	23.1	22.2
Stockholders' equity
Common stock, par value	4.6	4.5
Additional paid-in capital	9,771.5	9,730.9
Retained earnings (accumulated deficit)	4,971.7	4,862.0
Accumulated other comprehensive income (loss)	621.0	640.3
Treasury stock, at cost	(5,645.3)	(5,554.4)
Total stockholders' equity attributable to Principal Financial Group, Inc.	9,723.6	9,683.4
Noncontrolling interest	103.0	20.0
Total stockholders' equity	9,826.6	9,703.4	9,660.0	9,306.2
Total liabilities and stockholders' equity	200,762.9	161,830.2
Series A
Stockholders' equity
Preferred stock, value
Series B
Stockholders' equity
Preferred stock, value	0.1	0.1
Adjustments Related to Deferred Acquisition Cost Accounting Change
Assets
Fixed maturities, available-for-sale		50,939.3		49,006.7
Fixed maturities, trading		626.7		971.7
Equity securities, available-for-sale		136.5		77.1
Equity securities, trading		252.8		404.8
Mortgage loans		11,519.7		10,727.2
Real estate		1,180.3		1,092.9
Policy loans		864.9		885.1
Other investments		3,291.1		2,985.8
Total investments		68,811.3		66,151.3
Cash and cash equivalents		4,177.2		2,833.9
Accrued investment income		584.4		615.2
Premiums due and other receivables		1,084.4		1,196.5
Deferred acquisition costs		2,590.0		2,358.1
Property and equipment		464.2		457.2
Goodwill		543.4		482.3
Other intangibles		914.7		870.3
Separate account assets		81,653.8		71,364.4
Other assets		1,006.8		942.3
Total assets		161,830.2		147,271.5
Liabilities
Contractholder funds		37,786.5		37,676.4
Future policy benefits and claims		22,436.2		20,210.4
Other policyholder funds		716.4		548.6
Short-term debt		40.8		105.2
Long-term debt		2,671.3		1,564.8
Income taxes currently payable		15.3		3.1
Deferred income taxes		600.0		184.0
Separate account liabilities		81,653.8		71,364.4
Other liabilities		6,146.1		6,286.2
Total liabilities		152,066.4		137,943.1
Redeemable noncontrolling interest		60.4		22.2
Stockholders' equity
Common stock, par value		4.5		4.5
Additional paid-in capital		9,730.9		9,634.7
Retained earnings (accumulated deficit)		4,862.0		4,323.4
Accumulated other comprehensive income (loss)		640.3		271.4
Treasury stock, at cost		(5,554.4)		(5,281.7)
Total stockholders' equity attributable to Principal Financial Group, Inc.		9,683.4		8,952.4
Noncontrolling interest		20.0		353.8
Total stockholders' equity		9,703.4		9,306.2
Total liabilities and stockholders' equity		161,830.2		147,271.5
Adjustments Related to Deferred Acquisition Cost Accounting Change | Series A
Stockholders' equity
Preferred stock, value
Adjustments Related to Deferred Acquisition Cost Accounting Change | Series B
Stockholders' equity
Preferred stock, value		0.1		0.1
Adjustments Related to Deferred Acquisition Cost Accounting Change | As originally reported
Assets
Fixed maturities, available-for-sale		50,939.3		49,006.7
Fixed maturities, trading		626.7		971.7
Equity securities, available-for-sale		136.5		77.1
Equity securities, trading		252.8		404.8
Mortgage loans		11,519.7		10,727.2
Real estate		1,180.3		1,092.9
Policy loans		864.9		885.1
Other investments		3,291.1		2,985.8
Total investments		68,811.3		66,151.3
Cash and cash equivalents		4,177.2		2,833.9
Accrued investment income		584.4		615.2
Premiums due and other receivables		1,084.4		1,196.5
Deferred acquisition costs		2,673.8		2,428.0
Property and equipment		464.2		457.2
Goodwill		543.4		482.3
Other intangibles		927.2		890.6
Separate account assets		81,653.8		71,364.4
Other assets		1,006.8		942.3
Total assets		161,926.5		147,361.7
Liabilities
Contractholder funds		37,786.5		37,676.4
Future policy benefits and claims		22,436.2		20,210.4
Other policyholder funds		716.4		548.6
Short-term debt		40.8		105.2
Long-term debt		2,671.3		1,564.8
Income taxes currently payable		15.3		3.1
Deferred income taxes		626.5		208.7
Separate account liabilities		81,653.8		71,364.4
Other liabilities		6,146.1		6,286.2
Total liabilities		152,092.9		137,967.8
Redeemable noncontrolling interest		60.4		22.2
Stockholders' equity
Common stock, par value		4.5		4.5
Additional paid-in capital		9,730.9		9,634.7
Retained earnings (accumulated deficit)		4,940.2		4,402.3
Accumulated other comprehensive income (loss)		631.9		258.0
Treasury stock, at cost		(5,554.4)		(5,281.7)
Total stockholders' equity attributable to Principal Financial Group, Inc.		9,753.2		9,017.9
Noncontrolling interest		20.0		353.8
Total stockholders' equity		9,773.2		9,371.7
Total liabilities and stockholders' equity		161,926.5		147,361.7
Adjustments Related to Deferred Acquisition Cost Accounting Change | As originally reported | Series A
Stockholders' equity
Preferred stock, value
Adjustments Related to Deferred Acquisition Cost Accounting Change | As originally reported | Series B
Stockholders' equity
Preferred stock, value		$ 0.1		$ 0.1