Condensed Consolidating Financial Information - Shelf Registration Guarantor, Statements of Financial Position (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Assets
Fixed maturities, available-for-sale	$ 59,137.8	$ 54,846.1
Fixed maturities, trading	232.5	398.4
Equity securities, available-for-sale	118.3	98.9
Equity securities, trading	1,670.9	1,413.4
Mortgage loans	14,020.4	13,230.2
Real estate	1,650.2	1,368.8
Policy loans	812.0	823.8
Investment in unconsolidated entities	850.6	773.2
Other investments	2,710.7	2,882.7
Cash and cash equivalents	2,795.0	2,719.6	$ 2,218.2	$ 2,564.8
Accrued investment income	634.6	580.6
Premiums due and other receivables	1,516.6	1,361.9
Deferred acquisition costs	3,481.4	3,380.2
Property and equipment	754.6	699.0
Goodwill	1,055.2	1,020.8
Other intangibles	1,314.7	1,325.3
Separate account assets	154,085.9	139,832.6
Other assets	1,092.2	1,258.8
Total assets	247,933.6	228,014.3
Liabilities
Contractholder funds	38,208.6	37,953.6
Future policy benefits and claims	32,129.1	29,000.7
Other policyholder funds	926.2	890.4
Short-term debt	58.8	51.4
Long-term debt	3,175.4	3,125.7
Income taxes currently payable	53.7	12.9
Deferred income taxes	1,515.2	972.4
Separate account liabilities	154,085.9	139,832.6
Other liabilities	5,584.3	5,783.3
Total liabilities	235,737.2	217,623.0
Redeemable noncontrolling interest	129.1	97.5	189.9	85.7
Stockholders' equity
Common stock	4.7	4.7
Additional paid-in capital	9,886.8	9,686.0
Retained earnings (accumulated deficit)	8,784.6	7,720.4
Accumulated other comprehensive income (loss)	47.0	(675.2)
Treasury stock, at cost	(6,726.7)	(6,508.6)
Total stockholders' equity attributable to Principal Financial Group, Inc.	11,996.4	10,227.3
Noncontrolling interest	70.9	66.5
Total stockholders' equity	12,067.3	10,293.8	10,863.8	9,377.4
Total liabilities and stockholders' equity	247,933.6	228,014.3
Principal Financial Group, Inc. Parent Only
Assets
Investment in unconsolidated entities	14,631.1	12,597.9
Other investments	10.4	9.8
Cash and cash equivalents	618.1	882.6	761.7	578.7
Other assets	539.9	573.7
Total assets	15,799.5	14,064.0
Liabilities
Long-term debt	3,127.7	3,126.4
Other liabilities	675.4	710.3
Total liabilities	3,803.1	3,836.7
Stockholders' equity
Common stock	4.7	4.7
Additional paid-in capital	9,886.8	9,686.0
Retained earnings (accumulated deficit)	8,784.6	7,720.4
Accumulated other comprehensive income (loss)	47.0	(675.2)
Treasury stock, at cost	(6,726.7)	(6,508.6)
Total stockholders' equity attributable to Principal Financial Group, Inc.	11,996.4	10,227.3
Total stockholders' equity	11,996.4	10,227.3
Total liabilities and stockholders' equity	15,799.5	14,064.0
Principal Financial Services, Inc. Only
Assets
Equity securities, trading	5.6
Investment in unconsolidated entities	14,855.9	12,532.4
Other investments	105.5	135.9
Cash and cash equivalents	1,282.0	1,203.4	1,024.1	730.5
Accrued investment income		0.1
Premiums due and other receivables	0.5	0.3
Other assets	179.5	185.6
Total assets	16,429.0	14,057.7
Liabilities
Long-term debt	429.0	142.1
Other liabilities	1,467.5	1,411.3
Total liabilities	1,896.5	1,553.4
Stockholders' equity
Additional paid-in capital	9,037.8	9,010.9
Retained earnings (accumulated deficit)	5,037.1	3,724.3
Accumulated other comprehensive income (loss)	457.6	(230.9)
Total stockholders' equity attributable to Principal Financial Group, Inc.	14,532.5	12,504.3
Total stockholders' equity	14,532.5	12,504.3
Total liabilities and stockholders' equity	16,429.0	14,057.7
Principal Life Insurance Company and Other Subsidiaries Combined
Assets
Fixed maturities, available-for-sale	59,137.8	54,846.1
Fixed maturities, trading	232.5	398.4
Equity securities, available-for-sale	118.3	98.9
Equity securities, trading	1,665.3	1,413.4
Mortgage loans	14,020.4	13,230.2
Real estate	1,650.2	1,368.8
Policy loans	812.0	823.8
Investment in unconsolidated entities	746.2	697.5
Other investments	2,594.8	2,737.0
Cash and cash equivalents	2,561.4	2,114.8	1,871.5	2,413.3
Accrued investment income	634.6	580.5
Premiums due and other receivables	1,944.9	1,503.1
Deferred acquisition costs	3,481.4	3,380.2
Property and equipment	754.6	699.0
Goodwill	1,055.2	1,020.8
Other intangibles	1,314.7	1,325.3
Separate account assets	154,085.9	139,832.6
Other assets	1,479.9	1,200.9
Total assets	248,290.1	227,271.3
Liabilities
Contractholder funds	38,208.6	37,953.6
Future policy benefits and claims	32,129.1	29,000.7
Other policyholder funds	926.2	890.4
Short-term debt	58.8	127.9
Long-term debt	47.7	(0.8)
Income taxes currently payable	171.5	68.3
Deferred income taxes	2,511.3	1,619.3
Separate account liabilities	154,085.9	139,832.6
Other liabilities	5,035.5	4,962.1
Total liabilities	233,174.6	214,454.1
Redeemable noncontrolling interest	129.1	97.5
Stockholders' equity
Common stock	11.0	17.8
Additional paid-in capital	11,352.4	10,045.9
Retained earnings (accumulated deficit)	3,216.5	2,940.2
Accumulated other comprehensive income (loss)	337.6	(348.7)
Treasury stock, at cost	(2.0)	(2.0)
Total stockholders' equity attributable to Principal Financial Group, Inc.	14,915.5	12,653.2
Noncontrolling interest	70.9	66.5
Total stockholders' equity	14,986.4	12,719.7
Total liabilities and stockholders' equity	248,290.1	227,271.3
Principal Life Insurance Company and Other Subsidiaries Combined | Nonqualified benefit plans
Stockholders' equity
Assets held in Rabbi trusts	631.2	570.9
Liabilities held in Rabbi trusts	532.6	477.3
Eliminations, Shelf Registration Debt Guarantor
Assets
Investment in unconsolidated entities	(29,382.6)	(25,054.6)
Cash and cash equivalents	(1,666.5)	(1,481.2)	$ (1,439.1)	$ (1,157.7)
Premiums due and other receivables	(428.8)	(141.5)
Other assets	(1,107.1)	(701.4)
Total assets	(32,585.0)	(27,378.7)
Liabilities
Short-term debt		(76.5)
Long-term debt	(429.0)	(142.0)
Income taxes currently payable	(117.8)	(55.4)
Deferred income taxes	(996.1)	(646.9)
Other liabilities	(1,594.1)	(1,300.4)
Total liabilities	(3,137.0)	(2,221.2)
Stockholders' equity
Common stock	(11.0)	(17.8)
Additional paid-in capital	(20,390.2)	(19,056.8)
Retained earnings (accumulated deficit)	(8,253.6)	(6,664.5)
Accumulated other comprehensive income (loss)	(795.2)	579.6
Treasury stock, at cost	2.0	2.0
Total stockholders' equity attributable to Principal Financial Group, Inc.	(29,448.0)	(25,157.5)
Total stockholders' equity	(29,448.0)	(25,157.5)
Total liabilities and stockholders' equity	$ (32,585.0)	$ (27,378.7)