Segment Information - Narrative (Details) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2018 USD ($)		Jun. 30, 2017 USD ($)		Jun. 30, 2018 USD ($) segments		Jun. 30, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of segments | segments					2
Net results from structured products	$ 8,382	[1]	$ 2,923	[2]	$ 11,793	[3]	$ 5,837	[4]
Net fee income and other	(3,806)	[5]	(1,997)	[6]	(9,548)	[7]	218	[8]
Other operating expense
Segment Reporting Information [Line Items]
Net fee income and other	(12,100)		(12,200)		(24,600)		(23,700)
Total P&C | Net investment income
Segment Reporting Information [Line Items]
Net results from structured products	12,300		13,500		24,600		26,900
Total P&C | Interest expense
Segment Reporting Information [Line Items]
Net results from structured products	$ (3,900)		$ (10,600)		$ (12,800)		$ (21,100)

[1] The net results from P&C structured products include net investment income and interest expense of $12.3 million and $3.9 million, respectively.

[2] The net results from P&C structured products include net investment income and interest expense of $13.5 million and $10.6 million, respectively.

[3] The net results from P&C structured products include net investment income and interest expense of $24.6 million and $12.8 million, respectively.

[4] The net results from P&C structured products include net investment income and interest expense of $26.9 million and $21.1 million, respectively.

[5] Net fee income and other includes operating expenses of $12.1 million from the Company's loss prevention consulting services business.

[6] Net fee income and other includes operating expenses of $12.2 million from the Company's loss prevention consulting services business.

[7] Net fee income and other includes operating expenses of $24.6 million from the Company's loss prevention consulting services business.

[8] Net fee income and other includes operating expenses of $23.7 million from the Company's loss prevention consulting services business.