Reserves for Losses and Loss Settlement Expenses (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross liability for losses and loss settlement expenses at beginning of year	$ 1,224,183	$ 1,123,896
Ceded losses and loss settlement expenses	(59,871)	(59,794)
Net liability for losses and loss settlement expenses at beginning of year	1,164,312	1,064,102
Losses and loss settlement expenses incurred for claims occurring during
Current year	382,259	779,966
Prior years	(48,385)	(54,253)
Total incurred	333,874	725,713
Losses and loss settlement expense payments for claims occurring during
Current year	116,117	311,972
Prior years	197,750	313,531
Total paid	313,867	625,503
Net liability for losses and loss settlement expenses at end of year	1,184,319	1,164,312
Ceded loss and loss settlement expenses	56,528	59,871
Gross liability for losses and loss settlement expenses at end of period	$ 1,240,847	$ 1,224,183