Investments (Details 3) $ in Thousands	Jun. 30, 2018 USD ($) Holdings	Dec. 31, 2017 USD ($) Holdings
Fair value
Less than 12 months	$ 377,429
Less than 12 months		$ 282,770
12 months or longer	58,121
12 months or longer		66,019
Total	435,550
Total		348,789
Unrealized losses
Less than 12 months	6,317
Less than 12 months		2,238
12 months or longer	1,744
12 months or longer		1,407
Total	$ 8,061
Total		$ 3,645
No. of holdings | Holdings	634
No. of holdings | Holdings		417
Investment grade
Fair value
Less than 12 months	$ 269,062	$ 214,586
12 months or longer	54,423	62,193
Total	323,485	276,779
Unrealized losses
Less than 12 months	3,554	1,064
12 months or longer	1,577	985
Total	$ 5,131	$ 2,049
No. of holdings | Holdings	187	158
Non-investment grade
Fair value
Less than 12 months	$ 108,367	$ 57,447
12 months or longer	3,698	3,826
Total	112,065	61,273
Unrealized losses
Less than 12 months	2,763	1,074
12 months or longer	167	422
Total	$ 2,930	$ 1,496
No. of holdings | Holdings	447	253
Fixed maturities
Fair value
Less than 12 months	$ 377,429	$ 272,033
12 months or longer	58,121	66,019
Total	435,550	338,052
Unrealized losses
Less than 12 months	6,317	2,138
12 months or longer	1,744	1,407
Total	$ 8,061	$ 3,545
No. of holdings | Holdings	634	411
U.S. treasury
Fair value
Less than 12 months	$ 34,969	$ 10,237
12 months or longer	1,474	1,497
Total	36,443	11,734
Unrealized losses
Less than 12 months	278	110
12 months or longer	49	29
Total	$ 327	$ 139
No. of holdings | Holdings	5	4
States & political subdivisions
Fair value
Less than 12 months	$ 99,153	$ 52,553
12 months or longer	13,871	14,361
Total	113,024	66,914
Unrealized losses
Less than 12 months	1,763	288
12 months or longer	687	332
Total	$ 2,450	$ 620
No. of holdings | Holdings	57	33
Foreign government securities
Fair value
Less than 12 months	$ 499
12 months or longer	0
Total	499
Unrealized losses
Less than 12 months	1
12 months or longer	0
Total	$ 1
No. of holdings | Holdings	1
Corporate debt securities
Fair value
Less than 12 months	$ 194,322	$ 171,154
12 months or longer	28,430	31,113
Total	222,752	202,267
Unrealized losses
Less than 12 months	3,740	1,585
12 months or longer	556	339
Total	$ 4,296	$ 1,924
No. of holdings | Holdings	516	331
Residential mortgage-backed securities
Fair value
Less than 12 months	$ 4,526	$ 4,156
12 months or longer	5,416	7,064
Total	9,942	11,220
Unrealized losses
Less than 12 months	98	29
12 months or longer	146	95
Total	$ 244	$ 124
No. of holdings | Holdings	12	11
Commercial mortgage-backed securities
Fair value
Less than 12 months	$ 15,500	$ 10,836
12 months or longer	8,930	11,984
Total	24,430	22,820
Unrealized losses
Less than 12 months	301	85
12 months or longer	306	612
Total	$ 607	$ 697
No. of holdings | Holdings	22	19
Collateralized debt obligations
Fair value
Less than 12 months	$ 28,250	$ 21,598
12 months or longer	0	0
Total	28,250	21,598
Unrealized losses
Less than 12 months	136	41
12 months or longer	0	0
Total	$ 136	$ 41
No. of holdings | Holdings	20	12
Other debt securities
Fair value
Less than 12 months	$ 210	$ 1,499
12 months or longer	0	0
Total	210	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		75
12 months or longer		0
Total		$ 75
No. of holdings | Holdings		5
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		25
12 months or longer		0
Total		$ 25
No. of holdings | Holdings		1