Investments (Details 3) $ in Thousands	Mar. 31, 2018 USD ($) Holdings	Dec. 31, 2017 USD ($) Holdings
Fair value
Less than 12 months	$ 366,445	$ 282,770
12 months or longer	62,480	66,019
Total	428,925	348,789
Unrealized losses
Less than 12 months	5,915	2,238
12 months or longer	2,157	1,407
Total	$ 8,072	$ 3,645
No. of holdings | Holdings	558	417
Investment grade
Fair value
Less than 12 months	$ 286,647	$ 214,586
12 months or longer	58,721	62,193
Total	345,368	276,779
Unrealized losses
Less than 12 months	3,671	1,064
12 months or longer	1,693	985
Total	$ 5,364	$ 2,049
No. of holdings | Holdings	194	158
Non-investment grade
Fair value
Less than 12 months	$ 79,798	$ 57,447
12 months or longer	3,759	3,826
Total	83,557	61,273
Unrealized losses
Less than 12 months	2,244	1,074
12 months or longer	464	422
Total	$ 2,708	$ 1,496
No. of holdings | Holdings	364	253
Fixed maturities
Fair value
Less than 12 months	$ 366,445	$ 272,033
12 months or longer	62,480	66,019
Total	428,925	338,052
Unrealized losses
Less than 12 months	5,915	2,138
12 months or longer	2,157	1,407
Total	$ 8,072	$ 3,545
No. of holdings | Holdings	558	411
U.S. treasury
Fair value
Less than 12 months	$ 34,984	$ 10,237
12 months or longer	1,480	1,497
Total	36,464	11,734
Unrealized losses
Less than 12 months	232	110
12 months or longer	44	29
Total	$ 276	$ 139
No. of holdings | Holdings	5	4
States & political subdivisions
Fair value
Less than 12 months	$ 103,285	$ 52,553
12 months or longer	13,896	14,361
Total	117,181	66,914
Unrealized losses
Less than 12 months	1,934	288
12 months or longer	730	332
Total	$ 2,664	$ 620
No. of holdings | Holdings	59	33
Corporate debt securities
Fair value
Less than 12 months	$ 180,493	$ 171,154
12 months or longer	28,812	31,113
Total	209,305	202,267
Unrealized losses
Less than 12 months	3,290	1,585
12 months or longer	514	339
Total	$ 3,804	$ 1,924
No. of holdings | Holdings	438	331
Residential mortgage-backed securities
Fair value
Less than 12 months	$ 4,779	$ 4,156
12 months or longer	6,476	7,064
Total	11,255	11,220
Unrealized losses
Less than 12 months	94	29
12 months or longer	133	95
Total	$ 227	$ 124
No. of holdings | Holdings	14	11
Commercial mortgage-backed securities
Fair value
Less than 12 months	$ 15,108	$ 10,836
12 months or longer	11,816	11,984
Total	26,924	22,820
Unrealized losses
Less than 12 months	264	85
12 months or longer	736	612
Total	$ 1,000	$ 697
No. of holdings | Holdings	23	19
Collateralized debt obligations
Fair value
Less than 12 months	$ 27,463	$ 21,598
12 months or longer	0	0
Total	27,463	21,598
Unrealized losses
Less than 12 months	101	41
12 months or longer	0	0
Total	$ 101	$ 41
No. of holdings | Holdings	18	12
Other debt securities
Fair value
Less than 12 months	$ 333	$ 1,499
12 months or longer	0	0
Total	333	1,499
Unrealized losses
Less than 12 months	0	0
12 months or longer	0	0
Total	$ 0	$ 0
No. of holdings | Holdings	1	1
Nonredeemable preferred stock | Financial services sector
Fair value
Less than 12 months		$ 9,644
12 months or longer		0
Total		9,644
Unrealized losses
Less than 12 months		25
12 months or longer		0
Total		$ 25
No. of holdings | Holdings		1
Nonredeemable preferred stock | Utilities sector
Fair value
Less than 12 months		$ 1,093
12 months or longer		0
Total		1,093
Unrealized losses
Less than 12 months		75
12 months or longer		0
Total		$ 75
No. of holdings | Holdings		5