Investments - Gross Unrealized Losses for Fixed Maturities and Equity Securities (Details) $ in Millions	Jun. 30, 2017 USD ($) item	Dec. 31, 2016 USD ($) item
Fixed maturities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	$ 9,981.0	$ 14,955.3
Less than twelve months, Gross unrealized losses	176.1	396.6
Greater than or equal to twelve months, Fair value	1,959.4	2,511.1
Greater than or equal to twelve months, Gross unrealized losses	130.5	252.2
Total, Fair value	11,940.4	17,466.4
Total, Gross unrealized losses	306.6	648.8
U.S. government and agencies
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	379.8	570.3
Less than twelve months, Gross unrealized losses	7.1	10.9
Greater than or equal to twelve months, Fair value	10.1	8.2
Greater than or equal to twelve months, Gross unrealized losses	0.1
Total, Fair value	389.9	578.5
Total, Gross unrealized losses	7.2	10.9
Non-U.S. governments
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	59.0	198.0
Less than twelve months, Gross unrealized losses	0.8	5.4
Greater than or equal to twelve months, Fair value	10.8	12.2
Greater than or equal to twelve months, Gross unrealized losses	1.6	2.0
Total, Fair value	69.8	210.2
Total, Gross unrealized losses	2.4	7.4
States and political subdivisions
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	1,143.2	2,229.4
Less than twelve months, Gross unrealized losses	26.9	86.6
Greater than or equal to twelve months, Fair value	44.4	4.8
Greater than or equal to twelve months, Gross unrealized losses	2.3	0.5
Total, Fair value	1,187.6	2,234.2
Total, Gross unrealized losses	29.2	87.1
Corporate debt securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	3,825.5	6,559.7
Less than twelve months, Gross unrealized losses	78.8	189.2
Greater than or equal to twelve months, Fair value	919.9	1,285.6
Greater than or equal to twelve months, Gross unrealized losses	69.9	161.6
Total, Fair value	4,745.4	7,845.3
Total, Gross unrealized losses	148.7	350.8
Residential mortgage-backed pass-through securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	1,107.9	1,265.6
Less than twelve months, Gross unrealized losses	21.6	29.8
Greater than or equal to twelve months, Fair value	15.0	16.0
Greater than or equal to twelve months, Gross unrealized losses	0.8	0.8
Total, Fair value	1,122.9	1,281.6
Total, Gross unrealized losses	22.4	30.6
Commercial mortgage-backed securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	1,340.7	1,637.2
Less than twelve months, Gross unrealized losses	25.0	41.0
Greater than or equal to twelve months, Fair value	491.5	612.5
Greater than or equal to twelve months, Gross unrealized losses	24.9	46.9
Total, Fair value	1,832.2	2,249.7
Total, Gross unrealized losses	49.9	87.9
Collateralized debt obligations
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	434.6	265.7
Less than twelve months, Gross unrealized losses	0.6	0.9
Greater than or equal to twelve months, Fair value	94.4	195.6
Greater than or equal to twelve months, Gross unrealized losses	17.0	23.4
Total, Fair value	529.0	461.3
Total, Gross unrealized losses	17.6	24.3
Other debt obligations
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	1,690.3	2,229.4
Less than twelve months, Gross unrealized losses	15.3	32.8
Greater than or equal to twelve months, Fair value	373.3	376.2
Greater than or equal to twelve months, Gross unrealized losses	13.9	17.0
Total, Fair value	2,063.6	2,605.6
Total, Gross unrealized losses	29.2	49.8
Equity securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value		18.2
Less than twelve months, Gross unrealized losses		0.4
Greater than or equal to twelve months, Fair value	38.8	35.4
Greater than or equal to twelve months, Gross unrealized losses	7.1	10.5
Total, Fair value	38.8	53.6
Total, Gross unrealized losses	7.1	10.9
Principal Life Insurance Company | Fixed maturities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	9,765.6	14,549.4
Less than twelve months, Gross unrealized losses	170.6	384.6
Greater than or equal to twelve months, Fair value	1,811.7	2,369.5
Greater than or equal to twelve months, Gross unrealized losses	119.9	230.5
Total, Fair value	11,577.3	16,918.9
Total, Gross unrealized losses	$ 290.5	$ 615.1
Available-for-sale Securities, Continuous Unrealized Loss Position, Qualitative Disclosure
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Percent Investment Grade (as a percent)	94.00%	94.00%
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Average Price (percent of carrying value to amortized cost) | item	98	96
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less Than Twelve Months | item	1,289	1,911
Available-for-sale Securities in Unrealized Loss Positions, Average Price, Less Than Twelve Months (percent of carrying value to amortized cost) | item	98	97
Available-for-sale Securities in Unrealized Loss Positions, Percent Investment Grade, Less Than Twelve Months (as a percent)	97.00%	98.00%
Available-for-sale Securities in Unrealized Loss Position, Aggregate Losses On Investment Grade Investments, Less Than Twelve Months	$ 166.4	$ 374.1
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Twelve Months or Longer | item	361	453
Available-for-sale Securities in Unrealized Loss Positions, Average Price, Twelve Months or Longer (percent of carrying value to amortized cost) | item	94	91
Principal Life Insurance Company | Corporate debt securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Greater than or equal to twelve months, Gross unrealized losses	$ 60.9	$ 141.9
Available-for-sale Securities, Continuous Unrealized Loss Position, Qualitative Disclosure
Available-for-sale Securities in Unrealized Loss Positions, Average Price, Twelve Months or Longer (percent of carrying value to amortized cost) | item	93	89
Principal Life Insurance Company | Commercial mortgage-backed securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Greater than or equal to twelve months, Gross unrealized losses	$ 24.9	$ 46.9
Available-for-sale Securities, Continuous Unrealized Loss Position, Qualitative Disclosure
Available-for-sale Securities in Unrealized Loss Positions, Average Price, Twelve Months or Longer (percent of carrying value to amortized cost) | item	95	93