Reinsurance - Summary of Premiums Written, Premiums Earned, and Losses and Loss Adjustment Expenses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Written premiums:
Direct	$ 1,305,948	$ 1,027,222	$ 843,719
Assumed	164,787	139,551	238,186
Ceded	(574,585)	(404,101)	(315,279)
Net written premiums	896,150	762,672	766,626
Earned premiums:
Direct	1,182,501	990,221	842,182
Assumed	150,330	208,192	173,472
Ceded	(509,085)	(383,015)	(274,545)
Net earned premiums	823,746	815,398	741,109
Losses and loss adjustment expenses:
Direct	953,548	769,490	626,318
Assumed	113,843	131,346	142,818
Ceded	(395,289)	(300,560)	(213,759)
Net	$ 672,102	$ 600,276	$ 555,377