SUMMARY OF INVESTMENTS (Fair Value of Investments) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
HELD-TO-MATURITY
Amortized Cost	$ 150	$ 150
Fair Value	150	150
Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	2,991,387	2,956,009
Gross Unrealized Appreciation	295,163	247,298
Gross Unrealized Depreciation	16,889	34,765
Fair Value	3,269,661	3,168,542
U.S. Treasury | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	17,073	23,216
Gross Unrealized Appreciation	4	87
Gross Unrealized Depreciation	186	108
Fair Value	16,891	23,195
U.S. government agency | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	121,574	76,692
Gross Unrealized Appreciation	1,311	1,445
Gross Unrealized Depreciation	717	540
Fair Value	122,168	77,597
Fixed maturities
HELD-TO-MATURITY
Amortized Cost	150	150
Gross Unrealized Appreciation	0	0
Gross Unrealized Depreciation	0	0
Fair Value	150	150
AVAILABLE-FOR-SALE
Amortized Cost	1,516,610	1,458,235
Gross Unrealized Appreciation	27,412	18,725
Gross Unrealized Depreciation	8,952	23,674
Fair Value	1,535,070	1,453,286
Fixed maturities | Discontinued Operations
HELD-TO-MATURITY
Amortized Cost	34	48
Gross Unrealized Appreciation	0	1
Gross Unrealized Depreciation	0	0
Fair Value	34	49
AVAILABLE-FOR-SALE
Amortized Cost	1,412,291	1,429,270
Gross Unrealized Appreciation	25,055	26,115
Gross Unrealized Depreciation	7,321	10,545
Fair Value	1,430,025	1,444,840
Fixed maturities | Continuing and Discontinuing Operations
HELD-TO-MATURITY
Amortized Cost	184	198
Gross Unrealized Appreciation	0	1
Gross Unrealized Depreciation	0	0
Fair Value	184	199
AVAILABLE-FOR-SALE
Amortized Cost	2,928,901	2,887,505
Gross Unrealized Appreciation	52,467	44,840
Gross Unrealized Depreciation	16,273	34,219
Fair Value	2,965,095	2,898,126
General obligations | Midwest | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	107,689	143,747
Gross Unrealized Appreciation	2,446	1,808
Gross Unrealized Depreciation	439	1,412
Fair Value	109,696	144,143
General obligations | Northeast | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	47,477	57,731
Gross Unrealized Appreciation	1,174	909
Gross Unrealized Depreciation	10	231
Fair Value	48,641	58,409
General obligations | South | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	139,870	129,475
Gross Unrealized Appreciation	2,462	1,249
Gross Unrealized Depreciation	813	2,355
Fair Value	141,519	128,369
General obligations | West | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	111,123	114,524
Gross Unrealized Appreciation	2,351	1,380
Gross Unrealized Depreciation	463	2,173
Fair Value	113,011	113,731
Special revenue | Midwest | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	155,475	167,430
Gross Unrealized Appreciation	3,620	2,313
Gross Unrealized Depreciation	351	1,433
Fair Value	158,744	168,310
Special revenue | Northeast | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	79,028	70,202
Gross Unrealized Appreciation	1,351	487
Gross Unrealized Depreciation	619	2,624
Fair Value	79,760	68,065
Special revenue | South | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	260,145	244,225
Gross Unrealized Appreciation	5,218	1,753
Gross Unrealized Depreciation	1,851	6,791
Fair Value	263,512	239,187
Special revenue | West | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	156,576	134,287
Gross Unrealized Appreciation	2,929	1,509
Gross Unrealized Depreciation	1,198	4,052
Fair Value	158,307	131,744
Foreign bonds | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	51,361	62,995
Gross Unrealized Appreciation	1,441	2,239
Gross Unrealized Depreciation	49	0
Fair Value	52,753	65,234
Public utilities | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	206,028	212,360
Gross Unrealized Appreciation	3,386	3,761
Gross Unrealized Depreciation	270	447
Fair Value	209,144	215,674
Corporate bonds | Energy | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	93,191	107,084
Gross Unrealized Appreciation	1,972	2,195
Gross Unrealized Depreciation	110	419
Fair Value	95,053	108,860
Corporate bonds | Industrials | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	218,067	225,526
Gross Unrealized Appreciation	3,881	5,359
Gross Unrealized Depreciation	241	982
Fair Value	221,707	229,903
Corporate bonds | Consumer goods and services | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	183,253	178,135
Gross Unrealized Appreciation	3,498	3,847
Gross Unrealized Depreciation	494	295
Fair Value	186,257	181,687
Corporate bonds | Health care | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	74,125	81,211
Gross Unrealized Appreciation	1,312	2,063
Gross Unrealized Depreciation	29	151
Fair Value	75,408	83,123
Corporate bonds | Technology, media and telecommunications | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	146,853	143,402
Gross Unrealized Appreciation	2,376	2,029
Gross Unrealized Depreciation	250	819
Fair Value	148,979	144,612
Corporate bonds | Financial services | Continuing and Discontinuing Operations
HELD-TO-MATURITY
Amortized Cost	150	150
Gross Unrealized Appreciation	0	0
Gross Unrealized Depreciation	0	0
Fair Value	150	150
AVAILABLE-FOR-SALE
Amortized Cost	277,824	269,981
Gross Unrealized Appreciation	5,769	5,328
Gross Unrealized Depreciation	442	1,358
Fair Value	283,151	273,951
Mortgage-backed securities | Continuing and Discontinuing Operations
HELD-TO-MATURITY
Amortized Cost	34	48
Gross Unrealized Appreciation	0	1
Gross Unrealized Depreciation	0	0
Fair Value	34	49
AVAILABLE-FOR-SALE
Amortized Cost	13,828	17,288
Gross Unrealized Appreciation	101	201
Gross Unrealized Depreciation	238	241
Fair Value	13,691	17,248
Collateralized mortgage obligations | Government national mortgage association | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	157,836	145,947
Gross Unrealized Appreciation	1,921	1,279
Gross Unrealized Depreciation	2,274	2,766
Fair Value	157,483	144,460
Collateralized mortgage obligations | Federal home loan mortgage corporation | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	201,320	176,226
Gross Unrealized Appreciation	1,879	1,638
Gross Unrealized Depreciation	4,047	3,406
Fair Value	199,152	174,458
Collateralized mortgage obligations | Federal national mortgage association | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	104,903	101,414
Gross Unrealized Appreciation	1,703	1,816
Gross Unrealized Depreciation	1,174	1,334
Fair Value	105,432	101,896
Asset-backed securities | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	4,282	4,407
Gross Unrealized Appreciation	362	145
Gross Unrealized Depreciation	8	282
Fair Value	4,636	4,270
Equity securities
AVAILABLE-FOR-SALE
Amortized Cost	57,387	59,994
Gross Unrealized Appreciation	224,065	186,692
Gross Unrealized Depreciation	539	316
Fair Value	280,913	246,370
Equity securities | Discontinued Operations
AVAILABLE-FOR-SALE
Amortized Cost	5,099	8,510
Gross Unrealized Appreciation	18,631	15,766
Gross Unrealized Depreciation	77	230
Fair Value	23,653	24,046
Equity securities | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	62,486	68,504
Gross Unrealized Appreciation	242,696	202,458
Gross Unrealized Depreciation	616	546
Fair Value	304,566	270,416
Common stock | Public utilities | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	6,394	6,394
Gross Unrealized Appreciation	16,075	13,465
Gross Unrealized Depreciation	30	188
Fair Value	22,439	19,671
Common stock | Energy | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	6,514	6,514
Gross Unrealized Appreciation	8,171	8,555
Gross Unrealized Depreciation	120	22
Fair Value	14,565	15,047
Common stock | Industrials | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	13,117	13,252
Gross Unrealized Appreciation	53,522	38,715
Gross Unrealized Depreciation	120	173
Fair Value	66,519	51,794
Common stock | Consumer goods and services | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	10,110	10,324
Gross Unrealized Appreciation	15,742	13,851
Gross Unrealized Depreciation	164	58
Fair Value	25,688	24,117
Common stock | Health care | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	7,763	7,763
Gross Unrealized Appreciation	32,340	19,657
Gross Unrealized Depreciation	0	0
Fair Value	40,103	27,420
Common stock | Technology, media and telecommunications | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	6,067	5,931
Gross Unrealized Appreciation	11,556	9,476
Gross Unrealized Depreciation	115	38
Fair Value	17,508	15,369
Common stock | Financial services | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	11,529	17,289
Gross Unrealized Appreciation	104,985	98,728
Gross Unrealized Depreciation	67	67
Fair Value	116,447	115,950
Nonredeemable preferred stocks | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	992	1,037
Gross Unrealized Appreciation	305	11
Gross Unrealized Depreciation	0	0
Fair Value	$ 1,297	$ 1,048