Reserves for Losses and Loss Settlement Expenses (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Insurance Loss Reserves [Abstract]
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	182,783	779,966
Prior years	(38,055)	(54,253)
Total incurred	144,728	725,713
Losses and loss settlement expense payments for claims occurring during
Current year	44,734	311,972
Prior years	104,319	313,531
Total paid	149,053	625,503
Net liability for losses and loss settlement expenses at end of year	1,159,987	1,164,312
Ceded loss and loss settlement expenses	59,994	59,871
Gross liability for losses and loss settlement expenses at end of period	$ 1,219,981	$ 1,224,183