SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Reclassifications in Accumulated Other Comprehensive Earnings (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Amount Reclassified from Accumulated Other Comprehensive Earnings
Net realized gains									$ 34,740	$ 39,829	$ 32,182
Other-than-temporary-impairment losses on investments									(95)	0	0
Earnings before income taxes	$ 38,006	$ 31,142	$ 42,341	$ 45,593	$ 45,582	$ 51,937	$ 54,650	$ 44,513	157,082	196,682	189,487
Income tax expense									(42,162)	(59,138)	(54,042)
Net earnings	$ 32,187	$ 22,263	$ 29,077	$ 31,393	$ 33,853	$ 35,908	$ 37,185	$ 30,598	114,920	137,544	135,445
Reclassifications out of accumulated other comprehensive earnings | Unrealized Gains and Losses on Available-for-Sale Securities
Amount Reclassified from Accumulated Other Comprehensive Earnings
Net realized gains									43,024	32,939	33,423
Other-than-temporary-impairment losses on investments									(95)
Earnings before income taxes									42,929	32,939	33,423
Income tax expense									(15,025)	(11,529)	(11,698)
Net earnings									$ 27,904	$ 21,410	$ 21,725