Other Operating Expenses and Other Expenses (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Other Operating Expenses And Other Expenses [Abstract]
Amortization of policy acquisition costs	$ 30,198	$ 27,638
Other underwriting expenses of the operating segments	17,154	14,794
Other operating expenses of the Corporate and Other segment	7,431	6,461
Total	$ 54,783	$ 48,893