Reserves for Losses and Loss Settlement Expenses (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross liability for losses and loss settlement expenses at beginning of year	$ 1,421,754	$ 1,312,483
Ceded losses and loss settlement expenses	(68,536)	(57,094)
Net liability for losses and loss settlement expenses at beginning of year	1,353,218	1,255,389
Losses and loss settlement expenses incurred for claims occurring during
Current year	656,207	835,507
Prior years	(30,038)	(5,335)
Total incurred	626,169	830,172
Losses and loss settlement expense payments for claims occurring during
Current year	263,898	333,975
Prior years	330,190	398,368
Total paid	594,088	732,343
Net liability for losses and loss settlement expenses at end of year	1,385,299	1,353,218
Ceded loss and loss settlement expenses	169,784	68,536
Gross liability for losses and loss settlement expenses at end of period	$ 1,555,083	$ 1,421,754