Pension Plans (Summary of Plan Assets Investment Measured At Fair Value) (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
U.S. Qualified Defined Benefit Plans [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	$ 472.8	$ 513.4	$ 542.0
U.S. Qualified Defined Benefit Plans [Member] | Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	397.6	432.5
U.S. Qualified Defined Benefit Plans [Member] | Money Market Funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	4.7	3.9
U.S. Qualified Defined Benefit Plans [Member] | Fixed Income Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	402.3	436.4
U.S. Qualified Defined Benefit Plans [Member] | Domestic
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	54.6	60.6
U.S. Qualified Defined Benefit Plans [Member] | International
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	15.9	16.4
U.S. Qualified Defined Benefit Plans [Member] | Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	70.5	77.0
Chaucer Pension Plan [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	122.9	130.3
Invested assets, fair value	123.5	131.8	$ 120.2
Chaucer Pension Plan [Member] | Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	38.7	40.9
Chaucer Pension Plan [Member] | Domestic
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	17.0	18.5
Chaucer Pension Plan [Member] | International
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	52.4	56.9
Chaucer Pension Plan [Member] | Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	69.4	75.4
Chaucer Pension Plan [Member] | Real Estate Funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	14.8	14.0
Level 1 | U.S. Qualified Defined Benefit Plans [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	15.0	5.2
Level 1 | U.S. Qualified Defined Benefit Plans [Member] | Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	10.3	1.3
Level 1 | U.S. Qualified Defined Benefit Plans [Member] | Money Market Funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	4.7	3.9
Level 1 | U.S. Qualified Defined Benefit Plans [Member] | Fixed Income Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	15.0	5.2
Level 2 | U.S. Qualified Defined Benefit Plans [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	426.1	477.5
Level 2 | U.S. Qualified Defined Benefit Plans [Member] | Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	355.6	400.5
Level 2 | U.S. Qualified Defined Benefit Plans [Member] | Fixed Income Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	355.6	400.5
Level 2 | U.S. Qualified Defined Benefit Plans [Member] | Domestic
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	54.6	60.6
Level 2 | U.S. Qualified Defined Benefit Plans [Member] | International
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	15.9	16.4
Level 2 | U.S. Qualified Defined Benefit Plans [Member] | Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	70.5	77.0
Level 2 | Chaucer Pension Plan [Member] | Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	38.7	40.9
Level 2 | Chaucer Pension Plan [Member] | Domestic
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	17.0	18.5
Level 2 | Chaucer Pension Plan [Member] | International
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	52.4	56.9
Level 2 | Chaucer Pension Plan [Member] | Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	69.4	75.4
Level 3 | U.S. Qualified Defined Benefit Plans [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	31.7	30.7
Level 3 | U.S. Qualified Defined Benefit Plans [Member] | Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	31.7	30.7
Level 3 | U.S. Qualified Defined Benefit Plans [Member] | Fixed Income Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	31.7	30.7
Level 3 | Chaucer Pension Plan [Member] | Real Estate Funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	14.8	14.0
Estimate of Fair Value, Fair Value Disclosure | Chaucer Pension Plan [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	122.9	130.3
Estimate of Fair Value, Fair Value Disclosure | Level 2 | Chaucer Pension Plan [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	108.1	116.3
Estimate of Fair Value, Fair Value Disclosure | Level 3 | Chaucer Pension Plan [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	$ 14.8	$ 14.0