Other Operating Expenses and Other Expenses (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Other Operating Expenses And Other Expenses [Abstract]
Amortization of policy acquisition costs	$ 28,672	$ 31,614	$ 58,870	$ 59,252
Other underwriting expenses of the operating segments	15,772	15,327	32,926	30,121
Other operating expenses of the Corporate and Other segment	7,307	6,095	14,738	12,556
Total	$ 51,751	$ 53,036	$ 106,534	$ 101,929