Summary of Significant Accounting Policies - Summary of Deferred Policy Acquisition Cost Amortization and Net Advertising Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Deferred policy acquisition cost amortization	$ 555,350	$ 562,545	$ 539,231
Advertising expenses	$ 37,400	$ 39,600	$ 44,000