SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Investment in Unconsolidated Investees (Details)	3 Months Ended								12 Months Ended
	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) item	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
INVESTMENT IN UNCONSOLIDATED INVESTEES:
Investment amount	$ 94,967,000				$ 90,067,000				$ 94,967,000	$ 90,067,000
Investee earnings recorded in income									16,056,000	17,224,000	$ 10,833,000
Reinsurance assumed									48,303,000	37,159,000	30,434,000
Net premiums earned	$ 204,002,000	$ 200,815,000	$ 196,522,000	$ 190,027,000	188,296,000	$ 182,025,000	$ 184,331,000	$ 183,285,000	791,366,000	737,937,000	728,608,000
Additional summarized financial information for investee
Dividends received									$ 9,900,000	0	9,900,000
Maui Jim Inc.
INVESTMENT IN UNCONSOLIDATED INVESTEES:
Equity ownership interest (as a percent)	40.00%								40.00%
Prime Holdings Insurance Services, Inc. (Prime)
INVESTMENT IN UNCONSOLIDATED INVESTEES:
Equity ownership interest (as a percent)	23.00%								23.00%
Maui Jim Inc.
INVESTMENT IN UNCONSOLIDATED INVESTEES:
Investment amount	$ 79,500,000				77,700,000				$ 79,500,000	77,700,000
Investee earnings recorded in income									12,500,000	14,400,000	9,700,000
Total investee net income									30,300,000	34,400,000	26,900,000
Additional summarized financial information for investee
Total assets	265,600,000				259,400,000				265,600,000	259,400,000
Total liabilities	90,400,000				88,000,000				90,400,000	88,000,000
Total equity	175,200,000				171,400,000				175,200,000	171,400,000
Undistributed earnings included in retained earnings	68,900,000								68,900,000
Dividends received									9,900,000	0	9,900,000
Prime Holdings Insurance Services, Inc. (Prime)
INVESTMENT IN UNCONSOLIDATED INVESTEES:
Investment amount	$ 15,400,000				$ 12,400,000				15,400,000	12,400,000
Investee earnings recorded in income									$ 3,600,000	2,800,000	1,100,000
Number of insurance carriers Company writes through | item									2
Equity ownership interest (as a percent)	23.00%								23.00%
Percentage of quota share reinsurance treaty, effective January 1, 2014									25.00%
Reinsurance assumed									$ 41,100,000	29,600,000	13,400,000
Net premiums earned									$ 34,200,000	$ 21,000,000	$ 11,400,000