Segment Information - Narrative (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) segment	Jun. 30, 2016 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment			4
Fee income in Excess and Surplus Lines segment	$ 4.2	$ 2.7	$ 8.1	$ 5.0
Casualty Reinsurance
Segment Reporting Information [Line Items]
Increase in underwriting expense	$ 2.0		$ 2.0