SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Reclassifications in Accumulated Other Comprehensive Earnings (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Amount Reclassified from Accumulated Other Comprehensive Earnings
Net realized gains									$ 39,829	$ 32,182	$ 22,036
Earnings before income taxes	$ 45,582	$ 51,937	$ 54,650	$ 44,513	$ 46,479	$ 48,594	$ 52,423	$ 41,991	196,682	189,487	175,666
Income tax expense									(59,138)	(54,042)	(49,411)
Net earnings	$ 33,853	$ 35,908	$ 37,185	$ 30,598	$ 37,497	$ 33,254	$ 35,725	$ 28,969	137,544	135,445	126,255
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									32,939	33,423	22,871
Earnings before income taxes									32,939	33,423	22,871
Income tax expense									(11,529)	(11,698)	(8,005)
Net earnings									$ 21,410	$ 21,725	$ 14,866