Investments (Details 3) $ in Thousands	Sep. 30, 2018 USD ($) Holdings	Dec. 31, 2017 USD ($) Holdings
Fair value
Less than 12 months	$ 354,583
Less than 12 months		$ 282,770
12 months or longer	89,440
12 months or longer		66,019
Total	444,023
Total		348,789
Unrealized losses
Less than 12 months	4,773
Less than 12 months		2,238
12 months or longer	2,836
12 months or longer		1,407
Total	$ 7,609
Total		$ 3,645
No. of holdings | Holdings	577
No. of holdings | Holdings		417
Investment grade
Fair value
Less than 12 months	$ 284,054	$ 214,586
12 months or longer	77,869	62,193
Total	361,923	276,779
Unrealized losses
Less than 12 months	3,207	1,064
12 months or longer	2,325	985
Total	$ 5,532	$ 2,049
No. of holdings | Holdings	213	158
Non-investment grade
Fair value
Less than 12 months	$ 70,529	$ 57,447
12 months or longer	11,571	3,826
Total	82,100	61,273
Unrealized losses
Less than 12 months	1,566	1,074
12 months or longer	511	422
Total	$ 2,077	$ 1,496
No. of holdings | Holdings	364	253
Fixed maturities
Fair value
Less than 12 months	$ 354,583	$ 272,033
12 months or longer	89,440	66,019
Total	444,023	338,052
Unrealized losses
Less than 12 months	4,773	2,138
12 months or longer	2,836	1,407
Total	$ 7,609	$ 3,545
No. of holdings | Holdings	577	411
U.S. treasury
Fair value
Less than 12 months	$ 24,910	$ 10,237
12 months or longer	11,515	1,497
Total	36,425	11,734
Unrealized losses
Less than 12 months	31	110
12 months or longer	343	29
Total	$ 374	$ 139
No. of holdings | Holdings	5	4
States & political subdivisions
Fair value
Less than 12 months	$ 104,325	$ 52,553
12 months or longer	23,592	14,361
Total	127,917	66,914
Unrealized losses
Less than 12 months	1,995	288
12 months or longer	1,144	332
Total	$ 3,139	$ 620
No. of holdings | Holdings	67	33
Corporate debt securities
Fair value
Less than 12 months	$ 145,419	$ 171,154
12 months or longer	39,278	31,113
Total	184,697	202,267
Unrealized losses
Less than 12 months	2,281	1,585
12 months or longer	895	339
Total	$ 3,176	$ 1,924
No. of holdings | Holdings	425	331
Residential mortgage-backed securities
Fair value
Less than 12 months	$ 790	$ 4,156
12 months or longer	1,703	7,064
Total	2,493	11,220
Unrealized losses
Less than 12 months	3	29
12 months or longer	27	95
Total	$ 30	$ 124
No. of holdings | Holdings	3	11
Commercial mortgage-backed securities
Fair value
Less than 12 months	$ 22,564	$ 10,836
12 months or longer	13,352	11,984
Total	35,916	22,820
Unrealized losses
Less than 12 months	216	85
12 months or longer	427	612
Total	$ 643	$ 697
No. of holdings | Holdings	30	19
Collateralized debt obligations
Fair value
Less than 12 months	$ 43,995	$ 21,598
12 months or longer	0	0
Total	43,995	21,598
Unrealized losses
Less than 12 months	207	41
12 months or longer	0	0
Total	$ 207	$ 41
No. of holdings | Holdings	33	12
Other debt securities
Fair value
Less than 12 months	$ 12,580	$ 1,499
12 months or longer	0	0
Total	12,580	1,499
Unrealized losses
Less than 12 months	40	0
12 months or longer	0	0
Total	$ 40	$ 0
No. of holdings | Holdings	14	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