Investments (Details 6) $ in Millions	Jun. 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	$ 8,520.9	$ 4,991.3
Less than twelve months, Gross unrealized losses	198.0	87.1
Greater than or equal to twelve months, Fair value	2,034.4	2,886.3
Greater than or equal to twelve months, Gross unrealized losses	222.8	251.7
Total, Fair value	10,555.3	7,877.6
Total, Gross unrealized losses	420.8	338.8
U.S. government and agencies
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	400.2	211.5
Less than twelve months, Gross unrealized losses	9.9	0.7
Greater than or equal to twelve months, Fair value	39.8	95.0
Greater than or equal to twelve months, Gross unrealized losses	0.6	2.2
Total, Fair value	440.0	306.5
Total, Gross unrealized losses	10.5	2.9
Non-U.S. governments
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	39.2	20.3
Less than twelve months, Gross unrealized losses	0.3	1.4
Greater than or equal to twelve months, Fair value	18.2	7.5
Greater than or equal to twelve months, Gross unrealized losses	1.9	0.2
Total, Fair value	57.4	27.8
Total, Gross unrealized losses	2.2	1.6
States and political subdivisions
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	992.2	208.1
Less than twelve months, Gross unrealized losses	34.6	0.7
Greater than or equal to twelve months, Fair value	26.2	210.5
Greater than or equal to twelve months, Gross unrealized losses	0.8	3.4
Total, Fair value	1,018.4	418.6
Total, Gross unrealized losses	35.4	4.1
Corporate debt securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	4,679.1	3,072.1
Less than twelve months, Gross unrealized losses	132.0	76.8
Greater than or equal to twelve months, Fair value	996.7	1,238.3
Greater than or equal to twelve months, Gross unrealized losses	135.5	139.1
Total, Fair value	5,675.8	4,310.4
Total, Gross unrealized losses	267.5	215.9
Residential mortgage-backed pass-through securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	474.1	18.0
Less than twelve months, Gross unrealized losses	7.0
Greater than or equal to twelve months, Fair value	158.0	395.3
Greater than or equal to twelve months, Gross unrealized losses	5.5	6.3
Total, Fair value	632.1	413.3
Total, Gross unrealized losses	12.5	6.3
Commercial mortgage-backed securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	720.8	375.3
Less than twelve months, Gross unrealized losses	7.3	3.0
Greater than or equal to twelve months, Fair value	328.3	395.0
Greater than or equal to twelve months, Gross unrealized losses	41.6	59.9
Total, Fair value	1,049.1	770.3
Total, Gross unrealized losses	48.9	62.9
Collateralized debt obligations
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	143.0	114.8
Less than twelve months, Gross unrealized losses	0.7	1.0
Greater than or equal to twelve months, Fair value	140.7	112.0
Greater than or equal to twelve months, Gross unrealized losses	20.5	19.6
Total, Fair value	283.7	226.8
Total, Gross unrealized losses	21.2	20.6
Other debt obligations
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	1,072.3	971.2
Less than twelve months, Gross unrealized losses	6.2	3.5
Greater than or equal to twelve months, Fair value	326.5	432.7
Greater than or equal to twelve months, Gross unrealized losses	16.4	21.0
Total, Fair value	1,398.8	1,403.9
Total, Gross unrealized losses	22.6	24.5
Equity securities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	40.7	10.0
Less than twelve months, Gross unrealized losses	0.3
Greater than or equal to twelve months, Fair value	34.9	36.0
Greater than or equal to twelve months, Gross unrealized losses	10.9	9.8
Total, Fair value	75.6	46.0
Total, Gross unrealized losses	11.2	9.8
Principal Life Insurance Company | Fixed maturities
Gross Unrealized Losses for Fixed Maturities and Equity Securities
Less than twelve months, Fair value	8,368.4	4,907.1
Less than twelve months, Gross unrealized losses	195.2	85.4
Greater than or equal to twelve months, Fair value	1,890.2	2,731.6
Greater than or equal to twelve months, Gross unrealized losses	198.1	225.4
Total, Fair value	10,258.6	7,638.7
Total, Gross unrealized losses	$ 393.3	$ 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	1,169	685
Available-for-sale Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Twelve Months or Longer | item	312	429
Available-for-sale Securities in Unrealized Loss Position, Aggregate Losses On Investment Grade Investments, Less Than Twelve Months	$ 149.1	$ 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	96	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	98	98
Available-for-sale Securities in Unrealized Loss Positions, Average Price, Twelve Months or Longer (percent of carrying value to amortized cost) | item	91	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	$ 112.8	$ 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	89	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	$ 41.6	$ 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	89	87