Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Available-for-sale securities
Amortized cost, fixed maturities	$ 732,618
Amortized cost	732,618	$ 754,636
Gross unrealized gains	5,258	7,722
Gross unrealized losses	8,072	3,645
Estimated fair value, fixed maturities	729,804
Estimated fair value	729,804	758,713
Fixed maturities
Available-for-sale securities
Amortized cost, fixed maturities		741,799
Gross unrealized gains, fixed maturities		7,707
Gross unrealized losses, fixed maturities		3,545
Estimated fair value, fixed maturities		745,961
U.S. treasury
Available-for-sale securities
Amortized cost, fixed maturities	36,740	11,873
Gross unrealized gains, fixed maturities	0	0
Gross unrealized losses, fixed maturities	276	139
Estimated fair value, fixed maturities	36,464	11,734
States & political subdivisions
Available-for-sale securities
Amortized cost, fixed maturities	260,153	254,533
Gross unrealized gains, fixed maturities	3,663	5,351
Gross unrealized losses, fixed maturities	2,664	620
Estimated fair value, fixed maturities	261,152	259,264
Foreign government securities
Available-for-sale securities
Amortized cost, fixed maturities	501	501
Gross unrealized gains, fixed maturities	0	2
Gross unrealized losses, fixed maturities	0	0
Estimated fair value, fixed maturities	501	503
Corporate debt securities
Available-for-sale securities
Amortized cost, fixed maturities	302,395	346,759
Gross unrealized gains, fixed maturities	1,008	1,688
Gross unrealized losses, fixed maturities	3,804	1,924
Estimated fair value, fixed maturities	299,599	346,523
Residential mortgage-backed securities
Available-for-sale securities
Amortized cost, fixed maturities	23,980	25,324
Gross unrealized gains, fixed maturities	357	371
Gross unrealized losses, fixed maturities	227	124
Estimated fair value, fixed maturities	24,110	25,571
Commercial mortgage-backed securities
Available-for-sale securities
Amortized cost, fixed maturities	34,663	33,475
Gross unrealized gains, fixed maturities	12	26
Gross unrealized losses, fixed maturities	1,000	697
Estimated fair value, fixed maturities	33,675	32,804
Collateralized debt obligations
Available-for-sale securities
Amortized cost, fixed maturities	71,200	57,838
Gross unrealized gains, fixed maturities	206	237
Gross unrealized losses, fixed maturities	101	41
Estimated fair value, fixed maturities	71,305	58,034
Other debt securities
Available-for-sale securities
Amortized cost, fixed maturities	2,986	11,496
Gross unrealized gains, fixed maturities	12	32
Gross unrealized losses, fixed maturities	0	0
Estimated fair value, fixed maturities	$ 2,998	11,528
Nonredeemable preferred stock \| Financial services sector
Available-for-sale securities
Amortized cost, equity securities		11,719
Gross unrealized gains, equity securities		15
Gross unrealized losses, equity securities		75
Estimated fair value, equity securities		11,659
Nonredeemable preferred stock \| Utilities sector
Available-for-sale securities
Amortized cost, equity securities		1,118
Gross unrealized gains, equity securities		0
Gross unrealized losses, equity securities		25
Estimated fair value, equity securities		$ 1,093

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