Investments - Gross Unrealized Losses for Fixed Maturities and Equity Securities (Details) $ in Millions	Sep. 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	$ 8,404.4	$ 14,955.3
Less than twelve months, Gross unrealized losses	93.6	396.6
Greater than or equal to twelve months, Fair value	3,537.4	2,511.1
Greater than or equal to twelve months, Gross unrealized losses	167.7	252.2
Total, Fair value	11,941.8	17,466.4
Total, Gross unrealized losses	261.3	648.8
U.S. government and agencies
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	421.1	570.3
Less than twelve months, Gross unrealized losses	4.2	10.9
Greater than or equal to twelve months, Fair value	56.6	8.2
Greater than or equal to twelve months, Gross unrealized losses	2.8
Total, Fair value	477.7	578.5
Total, Gross unrealized losses	7.0	10.9
Non-U.S. governments
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	55.3	198.0
Less than twelve months, Gross unrealized losses	0.2	5.4
Greater than or equal to twelve months, Fair value	17.3	12.2
Greater than or equal to twelve months, Gross unrealized losses	1.8	2.0
Total, Fair value	72.6	210.2
Total, Gross unrealized losses	2.0	7.4
States and political subdivisions
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	791.4	2,229.4
Less than twelve months, Gross unrealized losses	6.0	86.6
Greater than or equal to twelve months, Fair value	449.8	4.8
Greater than or equal to twelve months, Gross unrealized losses	15.8	0.5
Total, Fair value	1,241.2	2,234.2
Total, Gross unrealized losses	21.8	87.1
Corporate debt securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	3,305.4	6,559.7
Less than twelve months, Gross unrealized losses	53.0	189.2
Greater than or equal to twelve months, Fair value	1,363.9	1,285.6
Greater than or equal to twelve months, Gross unrealized losses	75.9	161.6
Total, Fair value	4,669.3	7,845.3
Total, Gross unrealized losses	128.9	350.8
Residential mortgage-backed pass-through securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	738.3	1,265.6
Less than twelve months, Gross unrealized losses	9.4	29.8
Greater than or equal to twelve months, Fair value	232.6	16.0
Greater than or equal to twelve months, Gross unrealized losses	7.4	0.8
Total, Fair value	970.9	1,281.6
Total, Gross unrealized losses	16.8	30.6
Commercial mortgage-backed securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	1,092.5	1,637.2
Less than twelve months, Gross unrealized losses	11.3	41.0
Greater than or equal to twelve months, Fair value	764.8	612.5
Greater than or equal to twelve months, Gross unrealized losses	34.5	46.9
Total, Fair value	1,857.3	2,249.7
Total, Gross unrealized losses	45.8	87.9
Collateralized debt obligations
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	260.2	265.7
Less than twelve months, Gross unrealized losses	0.7	0.9
Greater than or equal to twelve months, Fair value	44.2	195.6
Greater than or equal to twelve months, Gross unrealized losses	12.6	23.4
Total, Fair value	304.4	461.3
Total, Gross unrealized losses	13.3	24.3
Other debt obligations
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	1,740.2	2,229.4
Less than twelve months, Gross unrealized losses	8.8	32.8
Greater than or equal to twelve months, Fair value	608.2	376.2
Greater than or equal to twelve months, Gross unrealized losses	16.9	17.0
Total, Fair value	2,348.4	2,605.6
Total, Gross unrealized losses	25.7	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	39.6	35.4
Greater than or equal to twelve months, Gross unrealized losses	6.3	10.5
Total, Fair value	39.6	53.6
Total, Gross unrealized losses	6.3	10.9
Principal Life Insurance Company | Fixed maturities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	8,037.0	14,549.4
Less than twelve months, Gross unrealized losses	83.8	384.6
Greater than or equal to twelve months, Fair value	3,470.4	2,369.5
Greater than or equal to twelve months, Gross unrealized losses	162.2	230.5
Total, Fair value	11,507.4	16,918.9
Total, Gross unrealized losses	$ 246.0	$ 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)	96.00%	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	1,061	1,911
Available-for-sale Securities in Unrealized Loss Positions, Average Price, Less Than Twelve Months (percent of carrying value to amortized cost) | item	99	97
Available-for-sale Securities in Unrealized Loss Positions, Percent Investment Grade, Less Than Twelve Months (as a percent)	99.00%	98.00%
Available-for-sale Securities in Unrealized Loss Position, Aggregate Losses On Investment Grade Investments, Less Than Twelve Months	$ 81.8	$ 374.1
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Twelve Months or Longer | item	600	453
Available-for-sale Securities in Unrealized Loss Positions, Average Price, Twelve Months or Longer (percent of carrying value to amortized cost) | item	96	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	$ 72.3	$ 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	95	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	$ 34.1	$ 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	96	93