Segment Information - Narrative (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) segment	Sep. 30, 2016 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment			4
Fee income in Excess and Surplus Lines segment	$ 3,900	$ 2,100	$ 12,000	$ 7,200
Catastrophe losses	10,000		10,000
Casualty Reinsurance
Segment Reporting Information [Line Items]
Catastrophe losses	2,700		2,700
Increase (decrease) in underwriting expense	0		2,000
Increase in investment income	480
Excess and Surplus Lines
Segment Reporting Information [Line Items]
Catastrophe losses	7,300		7,300
Increase (decrease) in underwriting expense	$ (1,300)		$ 0