Investments - Cost and Fair Value of Available-For-Sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Available-for-sale securities
Amortized cost	$ 724,950
Estimated fair value	730,701
U.S. Treasury
Available-for-sale securities
Amortized cost		$ 208,610
Gross unrealized gains		18
Gross unrealized losses		216
Estimated fair value		208,412
States and political subdivisions
Available-for-sale securities
Amortized cost		157,003
Gross unrealized gains		2,020
Gross unrealized losses		0
Estimated fair value		159,023
Corporate debt securities
Available-for-sale securities
Amortized cost	450,295	259,362
Gross unrealized gains	6,289	139
Gross unrealized losses	1,704	9,554
Estimated fair value	454,880	249,947
Residential mortgage-backed securities
Available-for-sale securities
Amortized cost	124,337	4,603
Gross unrealized gains	1,056	38
Gross unrealized losses	50	32
Estimated fair value	125,343	4,609
Commercial mortgage-backed securities
Available-for-sale securities
Amortized cost	67,210	47,022
Gross unrealized gains	479	80
Gross unrealized losses	148	587
Estimated fair value	67,541	46,515
Collateralized debt obligations
Available-for-sale securities
Amortized cost	78,059	65,039
Gross unrealized gains	44	30
Gross unrealized losses	247	830
Estimated fair value	77,856	64,239
Other debt securities
Available-for-sale securities
Amortized cost	5,049	15,756
Gross unrealized gains	71	33
Gross unrealized losses	39	11
Estimated fair value	5,081	15,778
Available-for-sale securities
Available-for-sale securities
Amortized cost	724,950	757,395
Gross unrealized gains	7,939	2,358
Gross unrealized losses	2,188	11,230
Estimated fair value	$ 730,701	$ 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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Amortized cost of 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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