Reserves for Losses and Loss Settlement Expenses (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Insurance Loss Reserves [Abstract]
Gross liability for losses and loss settlement expenses at beginning of year	$ 1,123,896	$ 1,003,895
Ceded losses and loss settlement expenses	(59,794)	(54,653)
Net liability for losses and loss settlement expenses at beginning of year	1,064,102	949,242
Losses and loss settlement expenses incurred for claims occurring during
Current year	386,352	683,662
Prior years	(41,202)	(31,229)
Total incurred	345,150	652,433
Losses and loss settlement expense payments for claims occurring during
Current year	116,808	277,053
Prior years	175,126	260,520
Total paid	291,934	537,573
Net liability for losses and loss settlement expenses at end of year	1,117,318	1,064,102
Ceded loss and loss settlement expenses	63,160	59,794
Gross liability for losses and loss settlement expenses at end of period	$ 1,180,478	$ 1,123,896