Segment Information - Summary of Segment Results (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Gross written premiums	$ 283,841	$ 327,334
Net earned premiums	145,918	190,152
Underwriting profit of insurance segments	7,331	15,052
Net investment income	20,836	19,431
Interest expense	2,876	2,808
Segment revenues	110,284	214,127
Segment goodwill	181,831	181,831	$ 181,831
Segment assets	4,996,810	3,299,251	$ 5,024,405
Excess and Surplus Lines
Segment Reporting Information [Line Items]
Gross written premiums	136,197	186,549
Net earned premiums	99,739	141,672
Underwriting profit of insurance segments	8,112	13,102
Net investment income	7,941	5,544
Interest expense	0	0
Segment revenues	83,829	152,437
Segment goodwill	181,831	181,831
Segment assets	2,293,444	1,013,069
Specialty Admitted Insurance
Segment Reporting Information [Line Items]
Gross written premiums	102,802	102,953
Net earned premiums	13,283	12,360
Underwriting profit of insurance segments	(988)	1,623
Net investment income	928	897
Interest expense	0	0
Segment revenues	12,673	13,736
Segment goodwill	0	0
Segment assets	829,699	706,451
Casualty Reinsurance
Segment Reporting Information [Line Items]
Gross written premiums	44,842	37,832
Net earned premiums	32,896	36,120
Underwriting profit of insurance segments	207	327
Net investment income	11,604	11,172
Interest expense	0	0
Segment revenues	13,280	46,010
Segment goodwill	0	0
Segment assets	1,824,729	1,517,142
Corporate and Other
Segment Reporting Information [Line Items]
Gross written premiums	0	0
Net earned premiums	0	0
Underwriting profit of insurance segments	0	0
Net investment income	363	1,818
Interest expense	2,876	2,808
Segment revenues	502	1,944
Segment goodwill	0	0
Segment assets	$ 48,938	$ 62,589