Reserves for Unpaid Losses and Loss Adjustment Expenses (Incurred Loss and Allocated LAE, Net of Reinsurance - Specialty Products - Property) (Details) - Property - Specialty Products $ in Millions	Dec. 31, 2016 USD ($) reported_claim	Dec. 31, 2015 USD ($)	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	$ 405.2
2007
Claims Development [Line Items]
Incurred Loss and Allocated LAE, Net of Reinsurance	8.6	$ 8.6	$ 8.6	$ 8.7	$ 7.7	$ 7.6	$ 7.6	$ 9.1	$ 11.1	$ 12.0
Total IBNR plus expected development on reported claims	$ 0.0
Cumulative number of reported claims | reported_claim	250
2008
Claims Development [Line Items]
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 26.8	26.8	26.8	26.8	26.6	25.6	25.4	29.2	$ 31.3
Total IBNR plus expected development on reported claims	$ 0.0
Cumulative number of reported claims | reported_claim	3,466
2009
Claims Development [Line Items]
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 51.6	52.5	52.6	52.9	53.3	52.9	52.1	$ 55.2
Total IBNR plus expected development on reported claims	$ 0.0
Cumulative number of reported claims | reported_claim	8,768
2010
Claims Development [Line Items]
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 55.8	55.8	55.9	56.7	55.6	49.5	$ 51.0
Total IBNR plus expected development on reported claims	$ 0.0
Cumulative number of reported claims | reported_claim	10,170
2011
Claims Development [Line Items]
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 60.7	60.8	60.6	60.2	59.8	$ 58.7
Total IBNR plus expected development on reported claims	$ 0.2
Cumulative number of reported claims | reported_claim	11,121
2012
Claims Development [Line Items]
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 92.2	92.2	92.9	86.7	$ 79.9
Total IBNR plus expected development on reported claims	$ 0.2
Cumulative number of reported claims | reported_claim	12,405
2013
Claims Development [Line Items]
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 27.7	27.6	27.6	$ 36.4
Total IBNR plus expected development on reported claims	$ 0.3
Cumulative number of reported claims | reported_claim	5,267
2014
Claims Development [Line Items]
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 31.6	32.1	$ 37.5
Total IBNR plus expected development on reported claims	$ 0.4
Cumulative number of reported claims | reported_claim	1,227
2015
Claims Development [Line Items]
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 25.7	$ 28.3
Total IBNR plus expected development on reported claims	$ 1.1
Cumulative number of reported claims | reported_claim	2,186
2016
Claims Development [Line Items]
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 24.5
Total IBNR plus expected development on reported claims	$ 5.2
Cumulative number of reported claims | reported_claim	1,825