INCOME TAXES - Valuation allowance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Valuation allowance - beginning of year	$ 16,157
Valuation allowance - end of year	18,955	$ 16,157
Operating loss carryforwards
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Change in valuation allowance	198	(27,116)
Foreign tax credit
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Change in valuation allowance	(1,359)	267
U.K. branch assets and other foreign rate differentials
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Change in valuation allowance	(205)	1,006
U.S. alternative minimum tax credits
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Change in valuation allowance	(900)	900
Income tax expense
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Valuation allowance - beginning of year	16,157	41,100
Valuation allowance - end of year	13,891	16,157
Accumulated other comprehensive income
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Valuation allowance - beginning of year	0	0
Valuation allowance - end of year	5,064	0
Change in investment - related items
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Change in valuation allowance	$ 5,064	$ 0