Description of Business, Basis of Presentation, and Summary of Significant Accounting Policies - Schedule of Cumulative Effect Adjustment of Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Agent balances, due premiums and other receivables	$ 265,258		$ 229,522
Other liabilities	486,772		451,398
Deferred income tax liabilities	164,405		152,572
Retained earnings	$ 1,489,520		$ 1,375,090
Accounting Standards Update 2014-09 [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Agent balances, due premiums and other receivables		$ 45,730
Other liabilities		14,400
Deferred income tax liabilities		6,647
Retained earnings		$ 24,683