Pension Plans (Summary of Plan Assets Investment Measured At Fair Value) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Chaucer Pension Plan [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value		$ 144.6	$ 116.5
Pension Plans [Member] | Estimated Fair Value [Member] | US [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	$ 80.5	86.7
Pension Plans [Member] | Estimated Fair Value [Member] | Fixed Maturities | US [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	35.1	38.8
Pension Plans [Member] | Estimated Fair Value [Member] | Money Market Mutual Funds | US [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	9.1	5.3
Pension Plans [Member] | Estimated Fair Value [Member] | Mutual Fund | US [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	36.3	42.6
Level 1 [Member] | Pension Plans [Member] | Estimated Fair Value [Member] | US [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	54.7	61.0
Level 1 [Member] | Pension Plans [Member] | Estimated Fair Value [Member] | Fixed Maturities | US [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	9.3	13.1
Level 1 [Member] | Pension Plans [Member] | Estimated Fair Value [Member] | Money Market Mutual Funds | US [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	9.1	5.3
Level 1 [Member] | Pension Plans [Member] | Estimated Fair Value [Member] | Mutual Fund | US [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	36.3	42.6
Level 3 [Member] | Pension Plans [Member] | Estimated Fair Value [Member] | US [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	25.8	25.7
Level 3 [Member] | Pension Plans [Member] | Estimated Fair Value [Member] | Fixed Maturities | US [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	$ 25.8	$ 25.7