Condensed Consolidating Financial Information - Guarantor, Statements of Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating activities
Net cash provided by (used in) operating activities	$ 4,377.1	$ 3,102.9	$ 2,221.2
Investing activities
Available-for-sale securities: Purchases	(9,920.3)	(9,054.0)	(9,025.2)
Available-for-sale securities: Sales	1,563.0	2,512.0	1,919.1
Available-for-sale securities: Maturities	6,625.9	6,244.7	7,359.2
Mortgage loans acquired or originated	(2,275.1)	(2,169.6)	(2,192.9)
Mortgage loans sold or repaid	1,687.3	1,793.6	2,095.1
Real estate acquired	(322.0)	(281.7)	(85.6)
Net (purchases) sales of property and equipment	(136.4)	(136.0)	(59.4)
Purchases of interest in subsidiaries, net of cash acquired	(291.2)		(1,268.3)
Net change in other investments	(98.8)	(81.7)	31.7
Net cash provided by (used in) investing activities	(3,167.6)	(1,172.7)	(1,226.3)
Financing activities
Issuance of common stock	76.1	77.5	125.8
Acquisition of treasury stock	(300.6)	(222.7)	(153.6)
Proceeds from financing element derivatives	0.3	15.1	47.0
Payments for financing element derivatives	(82.0)	(58.0)	(48.0)
Excess tax benefits from share-based payment arrangements	15.7	9.7	10.1
Purchase of subsidiary shares from noncontrolling interest	(22.5)	(227.0)	(52.9)
Sale of subsidiary shares to noncontrolling interest			31.8
Dividends to common stockholders	(441.0)	(376.6)	(288.4)
Dividends to preferred stockholders	(16.5)	(33.0)	(33.0)
Preferred stock redemption	(550.0)
Issuance of long-term debt	804.9	38.5	38.2
Principal repayments of long-term debt	(52.6)	(100.3)	(218.2)
Net proceeds from (repayments of) short-term borrowings	157.0	(118.3)	108.0
Investment contract deposits	6,492.3	5,638.4	6,716.3
Investment contract withdrawals	(6,666.8)	(7,099.2)	(8,852.7)
Net increase (decrease) in banking operation deposits	91.1	30.7	(225.7)
Other	(14.0)	(12.9)	(5.0)
Net cash provided by (used in) financing activities	(508.6)	(2,438.1)	(2,800.3)
Net increase (decrease) in cash and cash equivalents	700.9	(507.9)	(1,805.4)
Cash and cash equivalents at beginning of period	1,863.9	2,371.8	4,177.2
Cash and cash equivalents at end of period	2,564.8	1,863.9	2,371.8
Principal Financial Group, Inc. Parent Only
Operating activities
Net cash provided by (used in) operating activities	(85.1)	(81.6)	(42.8)
Investing activities
Dividends and returns of capital received from (contributions to) unconsolidated entities	685.5	917.7	319.5
Net change in other investments	5.4	(0.7)	(3.1)
Net cash provided by (used in) investing activities	690.9	917.0	316.4
Financing activities
Issuance of common stock	76.1	77.5	125.8
Acquisition of treasury stock	(300.6)	(222.7)	(153.6)
Excess tax benefits from share-based payment arrangements	0.7	0.3
Dividends to common stockholders	(441.0)	(376.6)	(288.4)
Dividends to preferred stockholders	(16.5)	(33.0)	(33.0)
Preferred stock redemption	(550.0)
Issuance of long-term debt	791.8
Net cash provided by (used in) financing activities	(439.5)	(554.5)	(349.2)
Net increase (decrease) in cash and cash equivalents	166.3	280.9	(75.6)
Cash and cash equivalents at beginning of period	412.4	131.5	207.1
Cash and cash equivalents at end of period	578.7	412.4	131.5
Principal Life Insurance Company Only
Operating activities
Net cash provided by (used in) operating activities	3,803.0	2,461.0	2,271.7
Investing activities
Available-for-sale securities: Purchases	(8,835.2)	(8,038.3)	(7,850.8)
Available-for-sale securities: Sales	1,017.2	2,066.1	1,536.0
Available-for-sale securities: Maturities	5,847.9	5,696.6	6,433.3
Mortgage loans acquired or originated	(2,177.1)	(2,141.0)	(2,378.4)
Mortgage loans sold or repaid	1,441.8	1,658.3	1,918.0
Real estate acquired	(0.3)	(0.8)
Net (purchases) sales of property and equipment	(109.3)	(115.1)	(52.4)
Dividends and returns of capital received from (contributions to) unconsolidated entities	(1.5)	255.2	444.2
Net change in other investments	555.0	175.7	56.1
Net cash provided by (used in) investing activities	(2,261.5)	(443.3)	106.0
Financing activities
Proceeds from financing element derivatives	0.3	15.1	47.0
Payments for financing element derivatives	(82.0)	(58.0)	(48.0)
Excess tax benefits from share-based payment arrangements	5.7	4.2	3.1
Principal repayments of long-term debt		(100.0)
Capital received from (dividends and capital paid to) parent	(485.6)	(867.7)	(249.5)
Investment contract deposits	6,214.8	5,349.1	6,355.1
Investment contract withdrawals	(6,655.5)	(7,088.8)	(8,846.6)
Other	(14.0)	(1.1)	(5.0)
Net cash provided by (used in) financing activities	(1,016.3)	(2,747.2)	(2,743.9)
Net increase (decrease) in cash and cash equivalents	525.2	(729.5)	(366.2)
Cash and cash equivalents at beginning of period	602.7	1,332.2	1,698.4
Cash and cash equivalents at end of period	1,127.9	602.7	1,332.2
Principal Financial Services, Inc. and Other Subsidiaries Combined
Operating activities
Net cash provided by (used in) operating activities	317.0	1,205.0	(170.1)
Investing activities
Available-for-sale securities: Purchases	(1,085.1)	(1,025.9)	(1,186.1)
Available-for-sale securities: Sales	546.4	445.9	434.0
Available-for-sale securities: Maturities	778.0	548.1	925.9
Mortgage loans acquired or originated	(325.8)	(188.6)	(202.2)
Mortgage loans sold or repaid	307.6	254.8	634.3
Real estate acquired	(321.7)	(280.9)	(85.6)
Net (purchases) sales of property and equipment	(27.1)	(20.9)	(7.0)
Purchases of interest in subsidiaries, net of cash acquired	(291.2)		(1,268.3)
Dividends and returns of capital received from (contributions to) unconsolidated entities	485.6	867.7	319.5
Net change in other investments	(319.8)	(290.7)	(70.2)
Net cash provided by (used in) investing activities	(253.1)	309.5	(505.7)
Financing activities
Excess tax benefits from share-based payment arrangements	9.3	5.2	7.0
Purchase of subsidiary shares from noncontrolling interest	(23.4)	(227.5)	(52.9)
Sale of subsidiary shares to noncontrolling interest	0.9	0.5	31.8
Issuance of long-term debt	235.5	140.9	38.2
Principal repayments of long-term debt	(116.4)	(81.1)	(214.4)
Net proceeds from (repayments of) short-term borrowings	157.0	(118.3)	108.0
Capital received from (dividends and capital paid to) parent	(684.0)	(1,172.9)	(763.7)
Investment contract deposits	277.5	289.3	361.2
Investment contract withdrawals	(11.3)	(10.4)	(6.1)
Net increase (decrease) in banking operation deposits	91.1	30.7	(225.7)
Other		(11.8)
Net cash provided by (used in) financing activities	(63.8)	(1,155.4)	(716.6)
Net increase (decrease) in cash and cash equivalents	0.1	359.1	(1,392.4)
Cash and cash equivalents at beginning of period	1,253.6	894.5	2,286.9
Cash and cash equivalents at end of period	1,253.7	1,253.6	894.5
Eliminations, Notes Guarantor
Operating activities
Net cash provided by (used in) operating activities	342.2	(481.5)	162.4
Investing activities
Available-for-sale securities: Purchases		10.2	11.7
Available-for-sale securities: Sales	(0.6)		(50.9)
Mortgage loans acquired or originated	227.8	160.0	387.7
Mortgage loans sold or repaid	(62.1)	(119.5)	(457.2)
Dividends and returns of capital received from (contributions to) unconsolidated entities	(1,169.6)	(2,040.6)	(1,083.2)
Net change in other investments	(339.4)	34.0	48.9
Net cash provided by (used in) investing activities	(1,343.9)	(1,955.9)	(1,143.0)
Financing activities
Purchase of subsidiary shares from noncontrolling interest	0.9	0.5
Sale of subsidiary shares to noncontrolling interest	(0.9)	(0.5)
Issuance of long-term debt	(222.4)	(102.4)
Principal repayments of long-term debt	63.8	80.8	(3.8)
Capital received from (dividends and capital paid to) parent	1,169.6	2,040.6	1,013.2
Net cash provided by (used in) financing activities	1,011.0	2,019.0	1,009.4
Net increase (decrease) in cash and cash equivalents	9.3	(418.4)	28.8
Cash and cash equivalents at beginning of period	(404.8)	13.6	(15.2)
Cash and cash equivalents at end of period	$ (395.5)	$ (404.8)	$ 13.6