Investments - Cost and Fair Value of AFS Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Available-for-sale securities
Amortized cost	$ 757,395	$ 741,799
Amortized cost		754,636
Gross unrealized gains	2,358	7,707
Gross unrealized gains		7,722
Gross unrealized losses	11,230	3,545
Gross unrealized losses		3,645
Estimated fair value	748,523	745,961
Estimated fair value		758,713
U.S. Treasury
Available-for-sale securities
Amortized cost	208,610	11,873
Gross unrealized gains	18	0
Gross unrealized losses	216	139
Estimated fair value	208,412	11,734
States and political subdivisions
Available-for-sale securities
Amortized cost	157,003	254,533
Gross unrealized gains	2,020	5,351
Gross unrealized losses	0	620
Estimated fair value	159,023	259,264
Foreign government securities
Available-for-sale securities
Amortized cost		501
Gross unrealized gains		2
Gross unrealized losses		0
Estimated fair value		503
Corporate debt securities
Available-for-sale securities
Amortized cost	259,362	346,759
Gross unrealized gains	139	1,688
Gross unrealized losses	9,554	1,924
Estimated fair value	249,947	346,523
Residential mortgage-backed securities
Available-for-sale securities
Amortized cost	4,603	25,324
Gross unrealized gains	38	371
Gross unrealized losses	32	124
Estimated fair value	4,609	25,571
Commercial mortgage-backed securities
Available-for-sale securities
Amortized cost	47,022	33,475
Gross unrealized gains	80	26
Gross unrealized losses	587	697
Estimated fair value	46,515	32,804
Collateralized debt obligations
Available-for-sale securities
Amortized cost	65,039	57,838
Gross unrealized gains	30	237
Gross unrealized losses	830	41
Estimated fair value	64,239	58,034
Other debt securities
Available-for-sale securities
Amortized cost	15,756	11,496
Gross unrealized gains	33	32
Gross unrealized losses	11	0
Estimated fair value	$ 15,778	11,528
Nonredeemable preferred stock \| Financial Services Sector
Available-for-sale securities
Amortized cost		11,719
Gross unrealized gains		15
Gross unrealized losses		75
Estimated fair value		11,659
Nonredeemable preferred stock \| Utilities Sector
Available-for-sale securities
Amortized cost		1,118
Gross unrealized gains		0
Gross unrealized losses		25
Estimated fair value		$ 1,093

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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