Condensed Consolidating Financial Information (Details 3) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
Operating activities
Net cash provided by (used in) operating activities	$ 2,117.4	$ 1,377.3
Investing activities
Available-for-sale securities: Purchases	(4,828.5)	(4,593.1)
Available-for-sale securities: Sales	983.1	1,385.6
Available-for-sale securities: Maturities	3,636.1	2,966.0
Mortgage loans acquired or originated	(1,057.1)	(956.9)
Mortgage loans sold or repaid	775.3	865.2
Real estate acquired	(204.1)	(193.9)
Net (purchases) sales of property and equipment	(67.3)	(93.3)
Net change in other investments	(174.6)	123.8
Net cash provided by (used in) investing activities	(937.1)	(496.6)
Financing activities
Issuance of common stock	29.6	26.1
Acquisition of treasury stock	(75.6)	(149.8)
Proceeds from financing element derivatives	0.2	14.6
Payments for financing element derivatives	(39.5)	(25.6)
Excess tax benefits from share-based payment arrangements	14.1	5.8
Purchase of subsidiary shares from noncontrolling interest	(11.0)	(40.1)
Dividends to common stockholders	(218.3)	(176.8)
Dividends to preferred stockholders	(16.5)	(16.5)
Preferred stock redemption	(550.0)
Issuance of long-term debt	797.4	27.4
Principal repayments of long-term debt	(19.8)	(100.1)
Net proceeds from (repayments of) short-term borrowings		(0.3)
Investment contract deposits	2,722.9	2,648.1
Investment contract withdrawals	(3,414.7)	(3,701.3)
Net increase (decrease) in banking operation deposits	8.2	(15.3)
Other	(6.8)	(6.4)
Net cash provided by (used in) financing activities	(779.8)	(1,510.2)
Net increase (decrease) in cash and cash equivalents	400.5	(629.5)
Cash and cash equivalents at beginning of period	1,863.9	2,371.8
Cash and cash equivalents at end of period	2,264.4	1,742.3
Principal Financial Group, Inc. Parent Only
Operating activities
Net cash provided by (used in) operating activities	(37.5)	(35.8)
Investing activities
Dividends and returns of capital received from (contributions to) unconsolidated entities	425.5	316.6
Net change in other investments	0.9	(4.8)
Net cash provided by (used in) investing activities	426.4	311.8
Financing activities
Issuance of common stock	29.6	26.1
Acquisition of treasury stock	(75.6)	(149.8)
Excess tax benefits from share-based payment arrangements	0.6	0.2
Dividends to common stockholders	(218.3)	(176.8)
Dividends to preferred stockholders	(16.5)	(16.5)
Preferred stock redemption	(550.0)
Issuance of long-term debt	791.8
Net cash provided by (used in) financing activities	(38.4)	(316.8)
Net increase (decrease) in cash and cash equivalents	350.5	(40.8)
Cash and cash equivalents at beginning of period	412.4	131.5
Cash and cash equivalents at end of period	762.9	90.7
Principal Life Insurance Company Only
Operating activities
Net cash provided by (used in) operating activities	1,901.3	1,043.2
Investing activities
Available-for-sale securities: Purchases	(4,332.1)	(4,125.3)
Available-for-sale securities: Sales	759.7	1,157.3
Available-for-sale securities: Maturities	3,226.9	2,711.9
Mortgage loans acquired or originated	(1,013.3)	(961.0)
Mortgage loans sold or repaid	663.8	864.8
Real estate acquired	(0.3)	(0.3)
Net (purchases) sales of property and equipment	(55.4)	(65.9)
Dividends and returns of capital received from (contributions to) unconsolidated entities	(2.8)	99.2
Net change in other investments	111.0	(43.0)
Net cash provided by (used in) investing activities	(642.5)	(362.3)
Financing activities
Proceeds from financing element derivatives	0.2	14.6
Payments for financing element derivatives	(39.5)	(25.6)
Excess tax benefits from share-based payment arrangements	5.1	2.4
Principal repayments of long-term debt		(100.0)
Capital received from (dividends and capital paid to) parent	(225.6)	(316.6)
Investment contract deposits	2,586.1	2,648.1
Investment contract withdrawals	(3,410.3)	(3,701.3)
Other	(6.8)	(6.5)
Net cash provided by (used in) financing activities	(1,090.8)	(1,484.9)
Net increase (decrease) in cash and cash equivalents	168.0	(804.0)
Cash and cash equivalents at beginning of period	602.7	1,332.2
Cash and cash equivalents at end of period	770.7	528.2
Principal Financial Services, Inc. and Other Subsidiaries Combined
Operating activities
Net cash provided by (used in) operating activities	209.1	288.2
Investing activities
Available-for-sale securities: Purchases	(496.4)	(478.6)
Available-for-sale securities: Sales	224.1	228.3
Available-for-sale securities: Maturities	409.2	254.1
Mortgage loans acquired or originated	(147.9)	(82.4)
Mortgage loans sold or repaid	139.6	87.1
Real estate acquired	(203.8)	(193.6)
Net (purchases) sales of property and equipment	(11.9)	(27.4)
Dividends and returns of capital received from (contributions to) unconsolidated entities	225.6	316.6
Net change in other investments	(188.9)	125.3
Net cash provided by (used in) investing activities	(50.4)	229.4
Financing activities
Issuance of common stock	0.1
Excess tax benefits from share-based payment arrangements	8.4	3.2
Purchase of subsidiary shares from noncontrolling interest	(11.6)	(40.1)
Sale of subsidiary shares to noncontrolling interest	0.6
Issuance of long-term debt	107.1	27.4
Principal repayments of long-term debt	(49.3)	(0.4)
Net proceeds from (repayments of) short-term borrowings		(0.3)
Capital received from (dividends and capital paid to) parent	(422.7)	(415.8)
Investment contract deposits	136.8
Investment contract withdrawals	(4.4)
Net increase (decrease) in banking operation deposits	8.2	(15.3)
Other		0.1
Net cash provided by (used in) financing activities	(226.8)	(441.2)
Net increase (decrease) in cash and cash equivalents	(68.1)	76.4
Cash and cash equivalents at beginning of period	1,253.6	894.5
Cash and cash equivalents at end of period	1,185.5	970.9
Eliminations, Notes Guarantor
Operating activities
Net cash provided by (used in) operating activities	44.5	81.7
Investing activities
Available-for-sale securities: Purchases		10.8
Available-for-sale securities: Sales	(0.7)
Mortgage loans acquired or originated	104.1	86.5
Mortgage loans sold or repaid	(28.1)	(86.7)
Dividends and returns of capital received from (contributions to) unconsolidated entities	(648.3)	(732.4)
Net change in other investments	(97.6)	46.3
Net cash provided by (used in) investing activities	(670.6)	(675.5)
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	(101.5)
Principal repayments of long-term debt	29.5	0.3
Capital received from (dividends and capital paid to) parent	648.3	732.4
Net cash provided by (used in) financing activities	576.2	732.7
Net increase (decrease) in cash and cash equivalents	(49.9)	138.9
Cash and cash equivalents at beginning of period	(404.8)	13.6
Cash and cash equivalents at end of period	$ (454.7)	$ 152.5