Condensed Consolidating Financial Information (Details 6) - 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 Financial Services, Inc. Only
Operating activities
Net cash provided by (used in) operating activities	69.0	(183.1)
Investing activities
Dividends and returns of capital received from (contributions to) unconsolidated entities	282.5	504.3
Net change in other investments	(28.9)	(8.5)
Net cash provided by (used in) investing activities	253.6	495.8
Financing activities
Capital received from (dividends and capital paid to) parent	(425.5)	(316.6)
Net cash provided by (used in) financing activities	(425.5)	(316.6)
Net increase (decrease) in cash and cash equivalents	(102.9)	(3.9)
Cash and cash equivalents at beginning of period	907.2	688.7
Cash and cash equivalents at end of period	804.3	684.8
Principal Life Insurance Company and Other Subsidiaries Combined
Operating activities
Net cash provided by (used in) operating activities	2,198.5	1,550.8
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	(163.5)	141.6
Net cash provided by (used in) investing activities	(926.0)	(478.8)
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	13.5	5.6
Purchase of subsidiary shares from noncontrolling interest	(11.0)	(40.1)
Issuance of long-term debt	5.6	27.4
Principal repayments of long-term debt	(19.8)	(100.1)
Net proceeds from (repayments of) short-term borrowings	0.4	(115.4)
Capital received from (dividends and capital paid to) parent	(282.5)	(504.3)
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	(1,023.5)	(1,812.8)
Net increase (decrease) in cash and cash equivalents	249.0	(740.8)
Cash and cash equivalents at beginning of period	1,598.0	2,384.0
Cash and cash equivalents at end of period	1,847.0	1,643.2
Eliminations, Shelf Registration Debt Guarantor
Operating activities
Net cash provided by (used in) operating activities	(112.6)	45.4
Investing activities
Dividends and returns of capital received from (contributions to) unconsolidated entities	(708.0)	(820.9)
Net change in other investments	16.9	(4.5)
Net cash provided by (used in) investing activities	(691.1)	(825.4)
Financing activities
Net proceeds from (repayments of) short-term borrowings	(0.4)	115.1
Capital received from (dividends and capital paid to) parent	708.0	820.9
Net cash provided by (used in) financing activities	707.6	936.0
Net increase (decrease) in cash and cash equivalents	(96.1)	156.0
Cash and cash equivalents at beginning of period	(1,053.7)	(832.4)
Cash and cash equivalents at end of period	$ (1,149.8)	$ (676.4)