SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Changes in Accumulated Other Comprehensive Earnings (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2018
COMPREHENSIVE EARNINGS
Tax rate used (as a percent)									21.00%	21.00%	35.00%
Other comprehensive income (loss), tax									$ 17,800,000	$ (9,000,000.0)	$ 19,000,000.0
Changes in the balance of each component of accumulated other comprehensive earnings
Beginning balance				$ (14,572,000)					(14,572,000)
Cumulative-effect adjustment from ASU 2016-01					$ 86,000					86,000
Net current-period other comprehensive earnings (loss)									67,045,000	(33,997,000)	35,309,000
Reclassification of stranded tax effect per ASU 2018-02										3,700,000
Ending balance	$ 52,473,000				(14,572,000)				52,473,000	(14,572,000)
Reclassification Adjustment Out of Accumulated Other Comprehensive Income
Net realized gains									17,520,000	63,624,000	6,970,000
Other-than-temporary-impairment (OTTI) losses on investments									0	(217,000)	(2,559,000)
Earnings before income taxes	63,218,000	$ 38,947,000	$ 48,828,000	81,741,000	(32,708,000)	$ 46,349,000	$ 39,562,000	$ 14,378,000	232,734,000	67,581,000	84,589,000
Income tax expense									(41,092,000)	(3,402,000)	20,439,000
Net earnings	53,378,000	$ 32,324,000	$ 40,467,000	65,473,000	(20,660,000)	$ 39,372,000	$ 33,251,000	12,216,000	191,642,000	64,179,000	105,028,000
Unrealized Gains and Losses on Available-for-Sale Securities
Changes in the balance of each component of accumulated other comprehensive earnings
Beginning balance				$ (14,572,000)				$ 157,919,000	(14,572,000)	157,919,000	122,610,000
Adjusted beginning balance	(14,572,000)				15,700,000				(14,572,000)	15,700,000	122,610,000
Other comprehensive earnings before reclassifications									69,560,000	(35,763,000)	40,887,000
Amounts reclassified from accumulated other comprehensive earnings									(2,515,000)	1,766,000	(5,578,000)
Net current-period other comprehensive earnings (loss)									67,045,000	(33,997,000)	35,309,000
Reclassification of stranded tax effect per ASU 2018-02										3,725,000
Ending balance	$ 52,473,000				(14,572,000)				52,473,000	(14,572,000)	157,919,000
Unrealized Gains and Losses on Available-for-Sale Securities | ASU 2016-01
Changes in the balance of each component of accumulated other comprehensive earnings
Cumulative-effect adjustment from ASU 2016-01					$ (142,219,000)					(142,219,000)		$ (142,200,000)
Reclassifications out of accumulated other comprehensive earnings
Reclassification Adjustment Out of Accumulated Other Comprehensive Income
Net realized gains									3,184,000	(2,018,000)	11,141,000
Other-than-temporary-impairment (OTTI) losses on investments										(217,000)	(2,559,000)
Earnings before income taxes									3,184,000	(2,235,000)	8,582,000
Income tax expense									(669,000)	469,000	(3,004,000)
Net earnings									$ 2,515,000	$ (1,766,000)	$ 5,578,000