Other Operating Expenses and Other Expenses (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Other Operating Expenses And Other Expenses [Abstract]
Amortization of policy acquisition costs	$ 35,311	$ 24,761	$ 94,563	$ 70,552
Other underwriting expenses of the operating segments	12,442	13,948	42,563	38,583
Other operating expenses of the Corporate and Other segment	6,507	4,870	19,063	15,597
Total	54,260	43,579	156,189	124,732
Other expenses	$ 119	$ (43)	$ 351	$ 36