SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Changes in Accumulated Other Comprehensive Earnings (Details) - USD ($)	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
COMPREHENSIVE EARNINGS
Tax rate used (as a percent)	21.00%		21.00%		21.00%	21.00%
Other comprehensive income (loss), tax	$ 14,200,000		$ 7,400,000		$ 10,800,000	$ 15,200,000
Changes in the balance of each component of accumulated other comprehensive earnings
Beginning balance		$ 52,473,000			52,473,000
Net current-period other comprehensive earnings (loss)	53,571,000	(13,031,000)	27,864,000	$ 29,301,000	40,540,000	57,165,000
Ending balance	93,035,000				93,035,000
Balance of securities for which an allowance for credit losses has been recognized in net earnings	1,752,000				1,752,000
Reclassification Adjustment Out of Accumulated Other Comprehensive Income
Net realized gains (losses)	(2,109,000)		4,764,000		13,043,000	13,832,000
Credit losses on available-for-sale securities	100,000				1,000,000.0
Earnings before income taxes	114,879,000		48,828,000		35,515,000	130,569,000
Income tax expense	(22,713,000)		(8,361,000)		(4,616,000)	(24,629,000)
Net earnings	92,166,000	(61,267,000)	40,467,000	65,473,000	30,899,000	105,940,000
Allowance for credit losses	985,000				985,000		$ 0
Debt Securities, Available-for-sale, Allowance for Credit Loss, Writeoff					0
Debt Securities, Available-for-sale, Allowance for Credit Loss, Recovery					0
Unrealized Gains and Losses on Available-for-Sale Securities
Changes in the balance of each component of accumulated other comprehensive earnings
Beginning balance	39,464,000	52,495,000	14,729,000	(14,572,000)	52,495,000	(14,572,000)
Other comprehensive earnings before reclassifications	53,733,000		28,388,000		41,853,000	58,183,000
Amounts reclassified from accumulated other comprehensive earnings	(162,000)		(524,000)		(1,313,000)	(1,018,000)
Net current-period other comprehensive earnings (loss)	53,571,000		27,864,000		40,540,000	57,165,000
Ending balance	93,035,000	39,464,000	42,593,000	14,729,000	93,035,000	42,593,000
Unrealized Gains and Losses on Available-for-Sale Securities | Previously Reported
Changes in the balance of each component of accumulated other comprehensive earnings
Beginning balance	39,464,000	52,473,000	14,729,000	(14,572,000)	52,473,000	(14,572,000)
Ending balance		39,464,000		$ 14,729,000
Unrealized Gains and Losses on Available-for-Sale Securities | Restatement Adjustment | Accounting Standards Update 2016-13
Changes in the balance of each component of accumulated other comprehensive earnings
Beginning balance		$ 22,000			22,000
Reclassifications out of accumulated other comprehensive earnings | Unrealized Gains and Losses on Available-for-Sale Securities
Reclassification Adjustment Out of Accumulated Other Comprehensive Income
Net realized gains (losses)	312,000		664,000		2,618,000	1,289,000
Credit losses on available-for-sale securities	107,000				956,000
Earnings before income taxes	205,000		664,000		1,662,000	1,289,000
Income tax expense	(43,000)		(140,000)		(349,000)	(271,000)
Net earnings	$ 162,000		$ 524,000		$ 1,313,000	$ 1,018,000