Investments - Cost to Fair Value of Available-For-Sale Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Available-for-sale securities
Amortized cost	$ 687,780
Estimated fair value	689,108
U.S. Treasury
Available-for-sale securities
Amortized cost	150,171	$ 208,610
Gross unrealized gains	923	18
Gross unrealized losses	0	216
Estimated fair value	151,094	208,412
States & political subdivisions
Available-for-sale securities
Amortized cost	3,354	157,003
Gross unrealized gains	6	2,020
Gross unrealized losses	0	0
Estimated fair value	3,360	159,023
Corporate debt securities
Available-for-sale securities
Amortized cost	356,899	259,362
Gross unrealized gains	1,706	139
Gross unrealized losses	1,949	9,554
Estimated fair value	356,656	249,947
Residential mortgage-backed securities
Available-for-sale securities
Amortized cost	65,910	4,603
Gross unrealized gains	297	38
Gross unrealized losses	21	32
Estimated fair value	66,186	4,609
Commercial mortgage-backed securities
Available-for-sale securities
Amortized cost	45,527	47,022
Gross unrealized gains	625	80
Gross unrealized losses	10	587
Estimated fair value	46,142	46,515
Collateralized debt obligations
Available-for-sale securities
Amortized cost	57,710	65,039
Gross unrealized gains	16	30
Gross unrealized losses	381	830
Estimated fair value	57,345	64,239
Other debt securities
Available-for-sale securities
Amortized cost	8,209	15,756
Gross unrealized gains	116	33
Gross unrealized losses	0	11
Estimated fair value	8,325	15,778
Available-for-sale securities
Available-for-sale securities
Amortized cost	687,780	757,395
Gross unrealized gains	3,689	2,358
Gross unrealized losses	2,361	11,230
Estimated fair value	$ 689,108	$ 748,523

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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