INVESTMENTS (NARRATIVE) (Details)	12 Months Ended
	Dec. 31, 2016 USD ($) investment state	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($) investment
Schedule of Available-for-sale Securities [Line Items]
Premium deficiencies adjustments to present value of future profits	$ (94,100,000)	$ (109,300,000)
Reduction to deferred acquisition costs due to unrealized gains that would result in premium deficiency if unrealized gains were realized	(96,400,000)	(33,200,000)
Adjustment to insurance liabilities	(13,500,000)	(14,600,000)
Increase to deferred tax assets due to unrealized gains that would result in premium deficiency if unrealized gains were realized	72,500,000	55,800,000
Fixed maturities, available for sale, amortized cost	19,803,100,000	18,947,000,000
Estimated fair value	21,096,200,000	19,882,900,000
Carrying value of fixed maturity investments and mortgage loans not accruing investment income	44,100,000	0
Net realized investment gains (losses)	8,300,000	(36,600,000)	$ 36,700,000
Net realized investment gains (losses), excluding impairment losses	47,900,000	(8,000,000)	64,000,000
Gain (loss) on dissolution of variable interest entities	(7,300,000)	11,300,000	0
Total other-than-temporary impairment losses	35,900,000	42,900,000	27,300,000
Other-than-temporary impairment losses recognized in accumulated other comprehensive income	3,600,000	3,000,000	$ 0
Number of legacy private company investments | investment			2
Net impairment losses recognized	$ (32,300,000)	(39,900,000)	$ (27,300,000)
Number of investments in default or considered nonperforming | investment	5
Fixed maturities in default, aggregate amortized cost	$ 19,200,000.0
Fixed maturities in default, carrying value	28,300,000
Value of available for sale securities sold	790,200,000	724,400,000	233,700,000
Impairment losses related to writedown of mortgage loans which losses major tenants			6,800,000
Other than temporary impairment losses, losses on other investments following unforeseen events	5,400,000	5,400,000	1,400,000
Other-than-temporary impairments included in accumulated other comprehensive income	8,000,000	4,900,000
Estimated fair value	$ 5,395,200,000
Percent of fixed maturities	25.60%
Number of additional states greater than specified percentage of mortgage loan balance | state	0
Assets held by insurance regulators	$ 36,700,000	38,200,000
Minimum [Member]
Schedule of Available-for-sale Securities [Line Items]
Percentage of mortgage loan balance	5.00%
Maximum [Member]
Schedule of Available-for-sale Securities [Line Items]
Percentage of commercial loan balance that is noncurrent	0.00%
California [Member]
Schedule of Available-for-sale Securities [Line Items]
Percentage of mortgage loan balance	14.00%
Texas [Member]
Schedule of Available-for-sale Securities [Line Items]
Percentage of mortgage loan balance	9.00%
Maryland [Member]
Schedule of Available-for-sale Securities [Line Items]
Percentage of mortgage loan balance	7.00%
Florida [Member]
Schedule of Available-for-sale Securities [Line Items]
Percentage of mortgage loan balance	6.00%
Total fixed maturities, available for sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Net realized investment gains (losses)	$ 30,900,000	(7,600,000)	51,400,000
Other-than-temporary impairment losses recognized in accumulated other comprehensive income	3,600,000	3,000,000	0
Net impairment losses recognized	(15,200,000)	(17,900,000)	0
Gross realized losses on sale	95,200,000	88,400,000	13,000,000
States and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross realized losses on sale			700,000
Corporate securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross realized losses on sale			12,300,000
Equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Net realized investment gains (losses)	20,900,000	3,700,000	10,100,000
Embedded Derivative Financial Instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Loss on embedded derivatives	400,000	9,200,000
Gain on embedded derivatives			7,600,000
Coinsurance [Member] | Embedded Derivative Financial Instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Loss on embedded derivatives		7,000,000
Gain on embedded derivatives	800,000		2,000,000
Marketable Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Net realized investment gains (losses), excluding impairment losses	47,500,000	8,200,000	54,400,000
Corporate securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross realized losses on sale	79,200,000
Fixed income investments [Member] | Energy Sector [Member]
Schedule of Available-for-sale Securities [Line Items]
Total other-than-temporary impairment losses	9,300,000	10,200,000
Gross realized losses on sale	63,500,000	59,700,000
Commercial bank loans held by VIEs [Member]
Schedule of Available-for-sale Securities [Line Items]
Total other-than-temporary impairment losses	1,200,000	16,400,000
Legacy private company investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Total other-than-temporary impairment losses		7,900,000	$ 19,100,000
Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross realized losses on sale	5,800,000
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross realized losses on sale	5,700,000
Other Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross realized losses on sale	4,500,000
Direct Loan [Member]
Schedule of Available-for-sale Securities [Line Items]
Total other-than-temporary impairment losses	3,700,000
Preferred Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Total other-than-temporary impairment losses	12,700,000
Non Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, available for sale, amortized cost	$ 3,070,000,000	2,234,500,000
Percentage of available-for-sale debt securities	16.00%
Estimated fair value	$ 3,147,800,000	2,222,800,000
Available-for-sale securities, percentage of amortized cost	103.00%
Other-than-temporary impairments included in accumulated other comprehensive income	$ 8,000,000	$ 4,900,000
Continuous unrealized loss position	20.00%