Condensed Consolidating Financial Information - Guarantor, Statements of Cash Flows (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015
Operating activities
Net cash provided by (used in) operating activities	$ 2,125.6	$ 2,117.4
Investing activities
Available-for-sale securities: Purchases	(7,476.9)	(4,828.5)
Available-for-sale securities: Sales	557.4	983.1
Available-for-sale securities: Maturities	3,667.9	3,636.1
Mortgage loans acquired or originated	(1,198.0)	(1,057.1)
Mortgage loans sold or repaid	910.4	775.3
Real estate acquired	(66.4)	(204.1)
Net (purchases) sales of property and equipment	(74.7)	(67.3)
Net change in other investments	6.3	(174.6)
Net cash provided by (used in) investing activities	(3,674.0)	(937.1)
Financing activities
Issuance of common stock	11.2	29.6
Acquisition of treasury stock	(209.7)	(75.6)
Proceeds from financing element derivatives	0.2	0.2
Payments for financing element derivatives	(43.0)	(39.5)
Excess tax benefits from share-based payment arrangements	5.5	14.1
Purchase of subsidiary shares from noncontrolling interest	(2.3)	(11.0)
Dividends to common stockholders	(223.2)	(218.3)
Dividends to preferred stockholders		(16.5)
Preferred stock redemption		(550.0)
Issuance of long-term debt	3.7	797.4
Principal repayments of long-term debt		(19.8)
Net proceeds from (repayments of) short-term borrowings	(156.9)
Investment contract deposits	6,877.7	2,722.9
Investment contract withdrawals	(5,071.0)	(3,414.7)
Net increase (decrease) in banking operation deposits	47.1	8.2
Other	(0.3)	(6.8)
Net cash provided by (used in) financing activities	1,239.0	(779.8)
Net increase (decrease) in cash and cash equivalents	(309.4)	400.5
Cash and cash equivalents at beginning of period	2,564.8	1,863.9
Cash and cash equivalents at end of period	2,255.4	2,264.4
Principal Financial Group, Inc. Parent Only
Operating activities
Net cash provided by (used in) operating activities	(54.6)	(37.5)
Investing activities
Dividends and returns of capital received from (contributed to) unconsolidated entities	594.8	425.5
Net change in other investments	(5.1)	0.9
Net cash provided by (used in) investing activities	589.7	426.4
Financing activities
Issuance of common stock	11.2	29.6
Acquisition of treasury stock	(209.7)	(75.6)
Excess tax benefits from share-based payment arrangements	0.2	0.6
Dividends to common stockholders	(223.2)	(218.3)
Dividends to preferred stockholders		(16.5)
Preferred stock redemption		(550.0)
Issuance of long-term debt		791.8
Net cash provided by (used in) financing activities	(421.5)	(38.4)
Net increase (decrease) in cash and cash equivalents	113.6	350.5
Cash and cash equivalents at beginning of period	578.7	412.4
Cash and cash equivalents at end of period	692.3	762.9
Principal Life Insurance Company Only
Operating activities
Net cash provided by (used in) operating activities	1,872.7	1,901.3
Investing activities
Available-for-sale securities: Purchases	(7,072.5)	(4,332.1)
Available-for-sale securities: Sales	295.3	759.7
Available-for-sale securities: Maturities	3,383.2	3,226.9
Mortgage loans acquired or originated	(1,123.0)	(1,013.3)
Mortgage loans sold or repaid	776.1	663.8
Real estate acquired		(0.3)
Net (purchases) sales of property and equipment	(54.8)	(55.4)
Dividends and returns of capital received from (contributed to) unconsolidated entities	1.5	(2.8)
Net change in other investments	186.9	111.0
Net cash provided by (used in) investing activities	(3,607.3)	(642.5)
Financing activities
Proceeds from financing element derivatives	0.2	0.2
Payments for financing element derivatives	(43.0)	(39.5)
Excess tax benefits from share-based payment arrangements	2.0	5.1
Capital received from (dividends and capital paid to) parent	(544.8)	(225.6)
Investment contract deposits	6,735.4	2,586.1
Investment contract withdrawals	(5,060.6)	(3,410.3)
Other	(0.5)	(6.8)
Net cash provided by (used in) financing activities	1,088.7	(1,090.8)
Net increase (decrease) in cash and cash equivalents	(645.9)	168.0
Cash and cash equivalents at beginning of period	1,127.9	602.7
Cash and cash equivalents at end of period	482.0	770.7
Principal Financial Services, Inc. and Other Subsidiaries Combined
Operating activities
Net cash provided by (used in) operating activities	882.8	209.1
Investing activities
Available-for-sale securities: Purchases	(409.7)	(496.4)
Available-for-sale securities: Sales	262.1	224.1
Available-for-sale securities: Maturities	284.7	409.2
Mortgage loans acquired or originated	(126.8)	(147.9)
Mortgage loans sold or repaid	137.6	139.6
Real estate acquired	(66.4)	(203.8)
Net (purchases) sales of property and equipment	(19.9)	(11.9)
Dividends and returns of capital received from (contributed to) unconsolidated entities	544.8	225.6
Net change in other investments	(656.3)	(188.9)
Net cash provided by (used in) investing activities	(49.9)	(50.4)
Financing activities
Issuance of common stock		0.1
Excess tax benefits from share-based payment arrangements	3.3	8.4
Purchase of subsidiary shares from noncontrolling interest	(2.3)	(11.6)
Sale of subsidiary shares to noncontrolling interest		0.6
Issuance of long-term debt	51.4	107.1
Principal repayments of long-term debt	(3.2)	(49.3)
Net proceeds from (repayments of) short-term borrowings	(156.9)
Capital received from (dividends and capital paid to) parent	(596.3)	(422.7)
Investment contract deposits	142.3	136.8
Investment contract withdrawals	(10.4)	(4.4)
Net increase (decrease) in banking operation deposits	47.1	8.2
Other	0.2
Net cash provided by (used in) financing activities	(524.8)	(226.8)
Net increase (decrease) in cash and cash equivalents	308.1	(68.1)
Cash and cash equivalents at beginning of period	1,253.7	1,253.6
Cash and cash equivalents at end of period	1,561.8	1,185.5
Eliminations, Notes Guarantor
Operating activities
Net cash provided by (used in) operating activities	(575.3)	44.5
Investing activities
Available-for-sale securities: Purchases	5.3
Available-for-sale securities: Sales		(0.7)
Mortgage loans acquired or originated	51.8	104.1
Mortgage loans sold or repaid	(3.3)	(28.1)
Dividends and returns of capital received from (contributed to) unconsolidated entities	(1,141.1)	(648.3)
Net change in other investments	480.8	(97.6)
Net cash provided by (used in) investing activities	(606.5)	(670.6)
Financing activities
Issuance of common stock		(0.1)
Purchase of subsidiary shares from noncontrolling interest		0.6
Sale of subsidiary shares to noncontrolling interest		(0.6)
Issuance of long-term debt	(47.7)	(101.5)
Principal repayments of long-term debt	3.2	29.5
Capital received from (dividends and capital paid to) parent	1,141.1	648.3
Net cash provided by (used in) financing activities	1,096.6	576.2
Net increase (decrease) in cash and cash equivalents	(85.2)	(49.9)
Cash and cash equivalents at beginning of period	(395.5)	(404.8)
Cash and cash equivalents at end of period	$ (480.7)	$ (454.7)