Reserves for Losses and Loss Settlement Expenses (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 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	181,497	683,662
Prior years	(24,946)	(31,229)
Total incurred	156,551	652,433
Losses and loss settlement expense payments for claims occurring during
Current year	39,100	277,053
Prior years	97,803	260,520
Total paid	136,903	537,573
Net liability for losses and loss settlement expenses at end of year	1,083,750	1,064,102
Ceded loss and loss settlement expenses	56,969	59,794
Gross liability for losses and loss settlement expenses at end of year	$ 1,140,719	$ 1,123,896