Reinsurance and Catastrophes - Effects of Reinsurance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Effects of reinsurance on premiums and benefits
Premiums written and contract deposits, Gross Amount	$ 1,244,500	$ 1,280,903	$ 1,277,066
Premiums written and contract deposits, Ceded to Other Companies	21,989	22,728	24,737
Premiums written and contract deposits, Assumed from Other Companies	4,606	4,324	4,184
Premiums written and contract deposits, Net Amount	1,227,117	1,262,499	1,256,513
Premiums and contract charges earned, Gross Amount	812,099	777,651	752,798
Premiums and contract charges earned, Ceded to Other Companies	22,036	22,826	25,077
Premiums and contract charges earned, Assumed from Other Companies	4,640	4,321	4,159
Premiums and contract charges earned, Net Amount	794,703	759,146	731,880
Benefits, claims and settlement expenses, Gross Amount	588,621	562,385	508,904
Benefits, claims and settlement expenses, Ceded to Other Companies	10,472	25,739	16,221
Benefits, claims and settlement expenses, Assumed from Other Companies	4,157	4,358	3,681
Benefits, claims and settlement expenses, Net Amount	$ 582,306	$ 541,004	$ 496,364