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, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Deferred policy acquisition cost amortization	$ 572,164	$ 555,350	$ 562,545
Net advertising expense	$ 40,900	$ 37,400	$ 39,600