Reserves for Losses and Loss Settlement Expenses (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross liability for losses and loss settlement expenses at beginning of year	$ 1,578,131	$ 1,421,754
Ceded losses and loss settlement expenses	(131,843)	(68,536)
Net liability for losses and loss settlement expenses at beginning of year	1,446,288	1,353,218
Losses and loss settlement expenses incurred for claims occurring during
Current year	560,080	887,119
Prior years	(26,099)	(17,652)
Total incurred	533,981	869,467
Losses and loss settlement expense payments for claims occurring during
Current year	204,056	354,635
Prior years	318,573	421,762
Total paid	522,629	776,397
Net liability for losses and loss settlement expenses at end of year	1,457,641	1,446,288
Ceded loss and loss settlement expenses	110,009	131,843
Gross liability for losses and loss settlement expenses at end of period	$ 1,567,650	$ 1,578,131