Reserves for Unpaid Losses and Loss Adjustment Expenses (Incurred Loss and Allocated LAE, Net of Reinsurance - Specialty Industries - Casualty) (Details) - Casualty - Specialty Industries $ in Millions	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2009 USD ($)	Dec. 31, 2008 USD ($)	Dec. 31, 2007 USD ($)
Claims Development [Line Items]
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 918.2
2007
Claims Development [Line Items]
Cumulative number of reported claims	1,770
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 36.9	$ 37.1	$ 36.0	$ 35.1	$ 35.1	$ 34.9	$ 34.0	$ 21.7	$ 24.3	$ 24.9
Total IBNR plus expected development on reported claims	$ 0.3
2008
Claims Development [Line Items]
Cumulative number of reported claims	1,835
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 22.1	22.2	23.7	25.2	25.8	26.4	30.4	26.2	$ 26.3
Total IBNR plus expected development on reported claims	$ 0.5
2009
Claims Development [Line Items]
Cumulative number of reported claims	2,982
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 51.0	51.6	53.6	54.8	49.6	51.5	51.1	$ 51.0
Total IBNR plus expected development on reported claims	$ 1.0
2010
Claims Development [Line Items]
Cumulative number of reported claims	4,791
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 77.2	75.5	73.3	69.4	69.3	72.4	$ 80.8
Total IBNR plus expected development on reported claims	$ 3.5
2011
Claims Development [Line Items]
Cumulative number of reported claims	5,254
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 112.2	111.8	100.5	90.2	87.5	$ 88.1
Total IBNR plus expected development on reported claims	$ 3.8
2012
Claims Development [Line Items]
Cumulative number of reported claims	6,298
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 98.8	101.8	102.0	103.4	$ 118.8
Total IBNR plus expected development on reported claims	$ 7.9
2013
Claims Development [Line Items]
Cumulative number of reported claims	7,107
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 138.2	133.6	131.5	$ 121.9
Total IBNR plus expected development on reported claims	$ 18.0
2014
Claims Development [Line Items]
Cumulative number of reported claims	7,633
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 124.2	127.4	$ 135.6
Total IBNR plus expected development on reported claims	$ 35.7
2015
Claims Development [Line Items]
Cumulative number of reported claims	7,315
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 127.7	$ 146.3
Total IBNR plus expected development on reported claims	$ 61.8
2016
Claims Development [Line Items]
Cumulative number of reported claims	5,701
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 129.9
Total IBNR plus expected development on reported claims	$ 98.4