Reserves for Losses and Loss Settlement Expenses (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross liability for losses and loss settlement expenses at beginning of year	$ 1,312,483	$ 1,224,183
Ceded losses and loss settlement expenses	(57,094)	(59,871)
Net liability for losses and loss settlement expenses at beginning of year	1,255,389	1,164,312
Losses and loss settlement expenses incurred for claims occurring during
Current year	168,889	785,778
Prior years	(4,649)	(54,167)
Total incurred	164,240	731,611
Losses and loss settlement expense payments for claims occurring during
Current year	41,136	306,032
Prior years	125,139	334,502
Total paid	166,275	640,534
Net liability for losses and loss settlement expenses at end of year	1,253,354	1,255,389
Ceded loss and loss settlement expenses	50,142	57,094
Gross liability for losses and loss settlement expenses at end of period	$ 1,303,496	$ 1,312,483