Investments (Details 6) $ in Millions	Sep. 30, 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	$ 8,357.4	$ 4,991.3
Less than twelve months, Gross unrealized losses	270.0	87.1
Greater than or equal to twelve months, Fair value	2,250.3	2,886.3
Greater than or equal to twelve months, Gross unrealized losses	296.2	251.7
Total, Fair value	10,607.7	7,877.6
Total, Gross unrealized losses	566.2	338.8
U.S. government and agencies
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	218.0	211.5
Less than twelve months, Gross unrealized losses	3.5	0.7
Greater than or equal to twelve months, Fair value	39.7	95.0
Greater than or equal to twelve months, Gross unrealized losses	0.2	2.2
Total, Fair value	257.7	306.5
Total, Gross unrealized losses	3.7	2.9
Non-U.S. governments
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	42.0	20.3
Less than twelve months, Gross unrealized losses	1.3	1.4
Greater than or equal to twelve months, Fair value	18.6	7.5
Greater than or equal to twelve months, Gross unrealized losses	1.9	0.2
Total, Fair value	60.6	27.8
Total, Gross unrealized losses	3.2	1.6
States and political subdivisions
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	582.4	208.1
Less than twelve months, Gross unrealized losses	20.1	0.7
Greater than or equal to twelve months, Fair value	6.4	210.5
Greater than or equal to twelve months, Gross unrealized losses	0.6	3.4
Total, Fair value	588.8	418.6
Total, Gross unrealized losses	20.7	4.1
Corporate debt securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	5,446.5	3,072.1
Less than twelve months, Gross unrealized losses	229.1	76.8
Greater than or equal to twelve months, Fair value	1,223.3	1,238.3
Greater than or equal to twelve months, Gross unrealized losses	210.1	139.1
Total, Fair value	6,669.8	4,310.4
Total, Gross unrealized losses	439.2	215.9
Residential mortgage-backed pass-through securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	260.3	18.0
Less than twelve months, Gross unrealized losses	1.6
Greater than or equal to twelve months, Fair value	154.8	395.3
Greater than or equal to twelve months, Gross unrealized losses	3.4	6.3
Total, Fair value	415.1	413.3
Total, Gross unrealized losses	5.0	6.3
Commercial mortgage-backed securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	682.6	375.3
Less than twelve months, Gross unrealized losses	7.2	3.0
Greater than or equal to twelve months, Fair value	296.0	395.0
Greater than or equal to twelve months, Gross unrealized losses	43.4	59.9
Total, Fair value	978.6	770.3
Total, Gross unrealized losses	50.6	62.9
Collateralized debt obligations
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	372.6	114.8
Less than twelve months, Gross unrealized losses	2.9	1.0
Greater than or equal to twelve months, Fair value	151.2	112.0
Greater than or equal to twelve months, Gross unrealized losses	21.2	19.6
Total, Fair value	523.8	226.8
Total, Gross unrealized losses	24.1	20.6
Other debt obligations
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	753.0	971.2
Less than twelve months, Gross unrealized losses	4.3	3.5
Greater than or equal to twelve months, Fair value	360.3	432.7
Greater than or equal to twelve months, Gross unrealized losses	15.4	21.0
Total, Fair value	1,113.3	1,403.9
Total, Gross unrealized losses	19.7	24.5
Equity securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	0.7	10.0
Less than twelve months, Gross unrealized losses	0.2
Greater than or equal to twelve months, Fair value	33.7	36.0
Greater than or equal to twelve months, Gross unrealized losses	12.1	9.8
Total, Fair value	34.4	46.0
Total, Gross unrealized losses	12.3	9.8
Principal Life Insurance Company | Fixed maturities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	7,986.8	4,907.1
Less than twelve months, Gross unrealized losses	263.6	85.4
Greater than or equal to twelve months, Fair value	2,113.2	2,731.6
Greater than or equal to twelve months, Gross unrealized losses	274.0	225.4
Total, Fair value	10,100.0	7,638.7
Total, Gross unrealized losses	$ 537.6	$ 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	1,038	685
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Twelve Months or Longer | item	356	429
Available-for-sale Securities in Unrealized Loss Position, Aggregate Losses On Investment Grade Investments, Less Than Twelve Months	$ 182.4	$ 44.4
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Percent Investment Grade (as a percent)	82.00%	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)	85.00%	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	89	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	$ 189.8	$ 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	85	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	$ 43.4	$ 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	87	87