Investments - Gross Unrealized Losses for Fixed Maturities and Equity Securities (Details) $ in Millions	Dec. 31, 2015 USD ($) item	Dec. 31, 2014 USD ($) item
Fixed maturities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	$ 14,421.5	$ 4,991.3
Less than twelve months, Gross unrealized losses	395.9	87.1
Greater than or equal to twelve months, Fair value	2,453.0	2,886.3
Greater than or equal to twelve months, Gross unrealized losses	427.3	251.7
Total, Fair value	16,874.5	7,877.6
Total, Gross unrealized losses	823.2	338.8
U.S. government and agencies
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	590.4	211.5
Less than twelve months, Gross unrealized losses	7.6	0.7
Greater than or equal to twelve months, Fair value	40.5	95.0
Greater than or equal to twelve months, Gross unrealized losses	0.7	2.2
Total, Fair value	630.9	306.5
Total, Gross unrealized losses	8.3	2.9
Non-U.S. governments
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	86.3	20.3
Less than twelve months, Gross unrealized losses	3.1	1.4
Greater than or equal to twelve months, Fair value	16.1	7.5
Greater than or equal to twelve months, Gross unrealized losses	1.9	0.2
Total, Fair value	102.4	27.8
Total, Gross unrealized losses	5.0	1.6
States and political subdivisions
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	692.0	208.1
Less than twelve months, Gross unrealized losses	19.0	0.7
Greater than or equal to twelve months, Fair value	6.5	210.5
Greater than or equal to twelve months, Gross unrealized losses	0.4	3.4
Total, Fair value	698.5	418.6
Total, Gross unrealized losses	19.4	4.1
Corporate debt securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	7,975.7	3,072.1
Less than twelve months, Gross unrealized losses	309.3	76.8
Greater than or equal to twelve months, Fair value	1,375.0	1,238.3
Greater than or equal to twelve months, Gross unrealized losses	328.9	139.1
Total, Fair value	9,350.7	4,310.4
Total, Gross unrealized losses	638.2	215.9
Residential mortgage-backed pass-through securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	656.7	18.0
Less than twelve months, Gross unrealized losses	6.7
Greater than or equal to twelve months, Fair value	147.9	395.3
Greater than or equal to twelve months, Gross unrealized losses	5.2	6.3
Total, Fair value	804.6	413.3
Total, Gross unrealized losses	11.9	6.3
Commercial mortgage-backed securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	1,480.8	375.3
Less than twelve months, Gross unrealized losses	27.3	3.0
Greater than or equal to twelve months, Fair value	299.5	395.0
Greater than or equal to twelve months, Gross unrealized losses	50.7	59.9
Total, Fair value	1,780.3	770.3
Total, Gross unrealized losses	78.0	62.9
Collateralized debt obligations
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	426.9	114.8
Less than twelve months, Gross unrealized losses	3.8	1.0
Greater than or equal to twelve months, Fair value	164.0	112.0
Greater than or equal to twelve months, Gross unrealized losses	22.8	19.6
Total, Fair value	590.9	226.8
Total, Gross unrealized losses	26.6	20.6
Other debt obligations
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	2,512.7	971.2
Less than twelve months, Gross unrealized losses	19.1	3.5
Greater than or equal to twelve months, Fair value	403.5	432.7
Greater than or equal to twelve months, Gross unrealized losses	16.7	21.0
Total, Fair value	2,916.2	1,403.9
Total, Gross unrealized losses	35.8	24.5
Equity securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	0.8	10.0
Less than twelve months, Gross unrealized losses	1.0
Greater than or equal to twelve months, Fair value	32.7	36.0
Greater than or equal to twelve months, Gross unrealized losses	13.2	9.8
Total, Fair value	33.5	46.0
Total, Gross unrealized losses	14.2	9.8
Principal Life Insurance Company | Fixed maturities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	13,673.9	4,907.1
Less than twelve months, Gross unrealized losses	376.3	85.4
Greater than or equal to twelve months, Fair value	2,306.1	2,731.6
Greater than or equal to twelve months, Gross unrealized losses	400.7	225.4
Total, Fair value	15,980.0	7,638.7
Total, Gross unrealized losses	$ 777.0	$ 310.8
Available-for-sale Securities, Continuous Unrealized Loss Position, Qualitative Disclosure
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less Than Twelve Months | item	1,725	685
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Twelve Months or Longer | item	404	429
Available-for-sale Securities in Unrealized Loss Position, Aggregate Losses On Investment Grade Investments, Less Than Twelve Months	$ 298.1	$ 44.4
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Percent Investment Grade (as a percent)	87.00%	80.00%
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Average Price (percent of carrying value to amortized cost) | item	95	96
Available-for-sale Securities in Unrealized Loss Positions, Percent Investment Grade, Less Than Twelve Months (as a percent)	90.00%	77.00%
Available-for-sale Securities in Unrealized Loss Positions, Average Price, Less Than Twelve Months (percent of carrying value to amortized cost) | item	97	98
Available-for-sale Securities in Unrealized Loss Positions, Average Price, Twelve Months or Longer (percent of carrying value to amortized cost) | item	85	92
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	$ 304.2	$ 113.0
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	80	91
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	$ 50.7	$ 59.9
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	86	87