SUMMARY OF INVESTMENTS (Investments in Unrealized Loss Position) (Details) $ in Thousands	Dec. 31, 2016 USD ($) issue	Dec. 31, 2015 USD ($) issue
AVAILABLE-FOR-SALE
Less than 12 months, number of issues | issue	567	378
Less than 12 months, fair value	$ 1,060,938	$ 747,083
Less than 12 months, gross unrealized depreciation	$ 32,167	$ 17,414
12 months or longer, number of issues | issue	55	92
12 months or longer, fair value	$ 53,798	$ 140,558
12 months or longer, gross unrealized depreciation	2,598	10,677
Total, fair value	1,114,736	887,641
Total, gross unrealized depreciation	$ 34,765	$ 28,091
Fixed Maturities
AVAILABLE-FOR-SALE
Less than 12 months, number of issues | issue	554	350
Less than 12 months, fair value	$ 1,060,577	$ 743,274
Less than 12 months, gross unrealized depreciation	$ 32,104	$ 16,962
12 months or longer, number of issues | issue	33	79
12 months or longer, fair value	$ 53,078	$ 140,088
12 months or longer, gross unrealized depreciation	2,115	10,140
Total, fair value	1,113,655	883,362
Total, gross unrealized depreciation	$ 34,219	$ 27,102
U.S. Treasury
AVAILABLE-FOR-SALE
Less than 12 months, number of issues | issue	9	6
Less than 12 months, fair value	$ 10,800	$ 6,408
Less than 12 months, gross unrealized depreciation	$ 108	$ 26
12 months or longer, number of issues | issue	0	2
12 months or longer, fair value	$ 0	$ 1,634
12 months or longer, gross unrealized depreciation	0	12
Total, fair value	10,800	8,042
Total, gross unrealized depreciation	$ 108	$ 38
U.S. government agency
AVAILABLE-FOR-SALE
Less than 12 months, number of issues | issue	10	38
Less than 12 months, fair value	$ 36,593	$ 104,621
Less than 12 months, gross unrealized depreciation	$ 540	$ 1,771
12 months or longer, number of issues | issue	0	6
12 months or longer, fair value	$ 0	$ 18,821
12 months or longer, gross unrealized depreciation	0	629
Total, fair value	36,593	123,442
Total, gross unrealized depreciation	$ 540	$ 2,400
General obligations | Midwest
AVAILABLE-FOR-SALE
Less than 12 months, number of issues | issue	27	4
Less than 12 months, fair value	$ 40,545	$ 2,417
Less than 12 months, gross unrealized depreciation	$ 1,412	$ 12
12 months or longer, number of issues | issue	0	1
12 months or longer, fair value	$ 0	$ 528
12 months or longer, gross unrealized depreciation	0	6
Total, fair value	40,545	2,945
Total, gross unrealized depreciation	$ 1,412	$ 18
General obligations | Northeast
AVAILABLE-FOR-SALE
Less than 12 months, number of issues | issue	9
Less than 12 months, fair value	$ 9,874
Less than 12 months, gross unrealized depreciation	$ 231
12 months or longer, number of issues | issue	0
12 months or longer, fair value	$ 0
12 months or longer, gross unrealized depreciation	0
Total, fair value	9,874
Total, gross unrealized depreciation	$ 231
General obligations | South
AVAILABLE-FOR-SALE
Less than 12 months, number of issues | issue	37	3
Less than 12 months, fair value	$ 53,699	$ 4,805
Less than 12 months, gross unrealized depreciation	$ 2,355	$ 55
12 months or longer, number of issues | issue	0	8
12 months or longer, fair value	$ 0	$ 3,743
12 months or longer, gross unrealized depreciation	0	79
Total, fair value	53,699	8,548
Total, gross unrealized depreciation	$ 2,355	$ 134
General obligations | West
AVAILABLE-FOR-SALE
Less than 12 months, number of issues | issue	30	4
Less than 12 months, fair value	$ 55,265	$ 8,927
Less than 12 months, gross unrealized depreciation	$ 2,173	$ 23
12 months or longer, number of issues | issue	0	2
12 months or longer, fair value	$ 0	$ 2,274
12 months or longer, gross unrealized depreciation	0	44
Total, fair value	55,265	11,201
Total, gross unrealized depreciation	$ 2,173	$ 67
Special revenue | Midwest
AVAILABLE-FOR-SALE
Less than 12 months, number of issues | issue	41	0
Less than 12 months, fair value	$ 62,937	$ 0
Less than 12 months, gross unrealized depreciation	$ 1,433	$ 0
12 months or longer, number of issues | issue	0	1
12 months or longer, fair value	$ 0	$ 2,494
12 months or longer, gross unrealized depreciation	0	30
Total, fair value	62,937	2,494
Total, gross unrealized depreciation	$ 1,433	$ 30
Special revenue | Northeast
AVAILABLE-FOR-SALE
Less than 12 months, number of issues | issue	22	1
Less than 12 months, fair value	$ 54,993	$ 4,755
Less than 12 months, gross unrealized depreciation	$ 2,624	$ 212
12 months or longer, number of issues | issue	0	0
12 months or longer, fair value	$ 0	$ 0
12 months or longer, gross unrealized depreciation	0	0
Total, fair value	54,993	4,755
Total, gross unrealized depreciation	$ 2,624	$ 212
Special revenue | South
AVAILABLE-FOR-SALE
Less than 12 months, number of issues | issue	79	4
Less than 12 months, fair value	$ 152,979	$ 7,445
Less than 12 months, gross unrealized depreciation	$ 6,791	$ 26
12 months or longer, number of issues | issue	0	2
12 months or longer, fair value	$ 0	$ 1,851
12 months or longer, gross unrealized depreciation	0	1
Total, fair value	152,979	9,296
Total, gross unrealized depreciation	$ 6,791	$ 27
Special revenue | West
AVAILABLE-FOR-SALE
Less than 12 months, number of issues | issue	44	4
Less than 12 months, fair value	$ 81,676	$ 6,851
Less than 12 months, gross unrealized depreciation	$ 4,052	$ 44
12 months or longer, number of issues | issue	0	0
12 months or longer, fair value	$ 0	$ 0
12 months or longer, gross unrealized depreciation	0	0
Total, fair value	81,676	6,851
Total, gross unrealized depreciation	$ 4,052	$ 44
Public utilities
AVAILABLE-FOR-SALE
Less than 12 months, number of issues | issue	20	35
Less than 12 months, fair value	$ 38,511	$ 72,680
Less than 12 months, gross unrealized depreciation	$ 423	$ 880
12 months or longer, number of issues | issue	2	5
12 months or longer, fair value	$ 2,122	$ 2,840
12 months or longer, gross unrealized depreciation	24	371
Total, fair value	40,633	75,520
Total, gross unrealized depreciation	$ 447	$ 1,251
Corporate bonds | Financial services
AVAILABLE-FOR-SALE
Less than 12 months, number of issues | issue	37	49
Less than 12 months, fair value	$ 75,286	$ 92,814
Less than 12 months, gross unrealized depreciation	$ 1,358	$ 1,143
12 months or longer, number of issues | issue	0	0
12 months or longer, fair value	$ 0	$ 0
12 months or longer, gross unrealized depreciation	0	0
Total, fair value	75,286	92,814
Total, gross unrealized depreciation	$ 1,358	$ 1,143
Corporate bonds | Energy
AVAILABLE-FOR-SALE
Less than 12 months, number of issues | issue	8	29
Less than 12 months, fair value	$ 15,938	$ 61,496
Less than 12 months, gross unrealized depreciation	$ 313	$ 3,286
12 months or longer, number of issues | issue	3	4
12 months or longer, fair value	$ 8,232	$ 7,991
12 months or longer, gross unrealized depreciation	106	1,427
Total, fair value	24,170	69,487
Total, gross unrealized depreciation	$ 419	$ 4,713
Corporate bonds | Industrials
AVAILABLE-FOR-SALE
Less than 12 months, number of issues | issue	24	38
Less than 12 months, fair value	$ 42,854	$ 78,588
Less than 12 months, gross unrealized depreciation	$ 596	$ 3,631
12 months or longer, number of issues | issue	3	3
12 months or longer, fair value	$ 5,641	$ 6,649
12 months or longer, gross unrealized depreciation	386	3,032
Total, fair value	48,495	85,237
Total, gross unrealized depreciation	$ 982	$ 6,663
Corporate bonds | Consumer goods and services
AVAILABLE-FOR-SALE
Less than 12 months, number of issues | issue	11	24
Less than 12 months, fair value	$ 21,059	$ 64,661
Less than 12 months, gross unrealized depreciation	$ 295	$ 770
12 months or longer, number of issues | issue	0	4
12 months or longer, fair value	$ 0	$ 2,491
12 months or longer, gross unrealized depreciation	0	6
Total, fair value	21,059	67,152
Total, gross unrealized depreciation	$ 295	$ 776
Corporate bonds | Health care
AVAILABLE-FOR-SALE
Less than 12 months, number of issues | issue	9	18
Less than 12 months, fair value	$ 20,918	$ 43,992
Less than 12 months, gross unrealized depreciation	$ 151	$ 652
12 months or longer, number of issues | issue	0	2
12 months or longer, fair value	$ 0	$ 3,737
12 months or longer, gross unrealized depreciation	0	139
Total, fair value	20,918	47,729
Total, gross unrealized depreciation	$ 151	$ 791
Corporate bonds | Technology, media and telecommunications
AVAILABLE-FOR-SALE
Less than 12 months, number of issues | issue	16	22
Less than 12 months, fair value	$ 41,230	$ 59,503
Less than 12 months, gross unrealized depreciation	$ 516	$ 1,478
12 months or longer, number of issues | issue	3	2
12 months or longer, fair value	$ 10,241	$ 8,940
12 months or longer, gross unrealized depreciation	303	525
Total, fair value	51,471	68,443
Total, gross unrealized depreciation	$ 819	$ 2,003
Mortgage-backed securities
AVAILABLE-FOR-SALE
Less than 12 months, number of issues | issue	16	9
Less than 12 months, fair value	$ 9,611	$ 7,423
Less than 12 months, gross unrealized depreciation	$ 187	$ 43
12 months or longer, number of issues | issue	5	4
12 months or longer, fair value	$ 1,198	$ 183
12 months or longer, gross unrealized depreciation	54	8
Total, fair value	10,809	7,606
Total, gross unrealized depreciation	$ 241	$ 51
Collateralized mortgage obligations | Government national mortgage association
AVAILABLE-FOR-SALE
Less than 12 months, number of issues | issue	36	17
Less than 12 months, fair value	$ 82,430	$ 29,769
Less than 12 months, gross unrealized depreciation	$ 2,261	$ 437
12 months or longer, number of issues | issue	9	14
12 months or longer, fair value	$ 13,603	$ 40,027
12 months or longer, gross unrealized depreciation	505	1,548
Total, fair value	96,033	69,796
Total, gross unrealized depreciation	$ 2,766	$ 1,985
Collateralized mortgage obligations | Federal home loan mortgage corporation
AVAILABLE-FOR-SALE
Less than 12 months, number of issues | issue	41	20
Less than 12 months, fair value	$ 105,775	$ 35,343
Less than 12 months, gross unrealized depreciation	$ 3,165	$ 644
12 months or longer, number of issues | issue	3	6
12 months or longer, fair value	$ 5,141	$ 19,887
12 months or longer, gross unrealized depreciation	241	698
Total, fair value	110,916	55,230
Total, gross unrealized depreciation	$ 3,406	$ 1,342
Collateralized mortgage obligations | Federal national mortgage association
AVAILABLE-FOR-SALE
Less than 12 months, number of issues | issue	27	15
Less than 12 months, fair value	$ 46,633	$ 32,800
Less than 12 months, gross unrealized depreciation	$ 1,091	$ 524
12 months or longer, number of issues | issue	4	11
12 months or longer, fair value	$ 4,341	$ 11,962
12 months or longer, gross unrealized depreciation	243	417
Total, fair value	50,974	44,762
Total, gross unrealized depreciation	$ 1,334	$ 941
Asset-backed securities
AVAILABLE-FOR-SALE
Less than 12 months, number of issues | issue	1	1
Less than 12 months, fair value	$ 971	$ 985
Less than 12 months, gross unrealized depreciation	$ 29	$ 16
12 months or longer, number of issues | issue	1	0
12 months or longer, fair value	$ 2,559	$ 0
12 months or longer, gross unrealized depreciation	253	0
Total, fair value	3,530	985
Total, gross unrealized depreciation	$ 282	$ 16
Foreign bonds
AVAILABLE-FOR-SALE
Less than 12 months, number of issues | issue		9
Less than 12 months, fair value		$ 16,991
Less than 12 months, gross unrealized depreciation		$ 1,289
12 months or longer, number of issues | issue		2
12 months or longer, fair value		$ 4,036
12 months or longer, gross unrealized depreciation		1,168
Total, fair value		21,027
Total, gross unrealized depreciation		$ 2,457
Equity securities
AVAILABLE-FOR-SALE
Less than 12 months, number of issues | issue	13	28
Less than 12 months, fair value	$ 361	$ 3,809
Less than 12 months, gross unrealized depreciation	$ 63	$ 452
12 months or longer, number of issues | issue	22	13
12 months or longer, fair value	$ 720	$ 470
12 months or longer, gross unrealized depreciation	483	537
Total, fair value	1,081	4,279
Total, gross unrealized depreciation	$ 546	$ 989
Common stock | Financial services
AVAILABLE-FOR-SALE
Less than 12 months, number of issues | issue	3	6
Less than 12 months, fair value	$ 53	$ 326
Less than 12 months, gross unrealized depreciation	$ 3	$ 51
12 months or longer, number of issues | issue	2	1
12 months or longer, fair value	$ 150	$ 136
12 months or longer, gross unrealized depreciation	64	58
Total, fair value	203	462
Total, gross unrealized depreciation	$ 67	$ 109
Common stock | Public utilities
AVAILABLE-FOR-SALE
Less than 12 months, number of issues | issue	0	0
Less than 12 months, fair value	$ 0	$ 0
Less than 12 months, gross unrealized depreciation	$ 0	$ 0
12 months or longer, number of issues | issue	3	3
12 months or longer, fair value	$ 120	$ 115
12 months or longer, gross unrealized depreciation	188	193
Total, fair value	120	115
Total, gross unrealized depreciation	$ 188	$ 193
Common stock | Energy
AVAILABLE-FOR-SALE
Less than 12 months, number of issues | issue	0	10
Less than 12 months, fair value	$ 0	$ 2,868
Less than 12 months, gross unrealized depreciation	$ 0	$ 266
12 months or longer, number of issues | issue	1	0
12 months or longer, fair value	$ 163	$ 0
12 months or longer, gross unrealized depreciation	22	0
Total, fair value	163	2,868
Total, gross unrealized depreciation	$ 22	$ 266
Common stock | Industrials
AVAILABLE-FOR-SALE
Less than 12 months, number of issues | issue	0	3
Less than 12 months, fair value	$ 0	$ 177
Less than 12 months, gross unrealized depreciation	$ 0	$ 44
12 months or longer, number of issues | issue	6	5
12 months or longer, fair value	$ 239	$ 193
12 months or longer, gross unrealized depreciation	173	269
Total, fair value	239	370
Total, gross unrealized depreciation	$ 173	$ 313
Common stock | Consumer goods and services
AVAILABLE-FOR-SALE
Less than 12 months, number of issues | issue	3	0
Less than 12 months, fair value	$ 282	$ 0
Less than 12 months, gross unrealized depreciation	$ 55	$ 0
12 months or longer, number of issues | issue	2	2
12 months or longer, fair value	$ 15	$ 14
12 months or longer, gross unrealized depreciation	3	3
Total, fair value	297	14
Total, gross unrealized depreciation	$ 58	$ 3
Common stock | Technology, media and telecommunications
AVAILABLE-FOR-SALE
Less than 12 months, number of issues | issue	7	9
Less than 12 months, fair value	$ 26	$ 438
Less than 12 months, gross unrealized depreciation	$ 5	$ 91
12 months or longer, number of issues | issue	8	2
12 months or longer, fair value	$ 33	$ 12
12 months or longer, gross unrealized depreciation	33	14
Total, fair value	59	450
Total, gross unrealized depreciation	$ 38	$ 105