Investments - Cost and Fair Value, Net of Credit Loss Allowance, of Available-For-Sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Available-for-sale securities
Amortized cost	$ 887,936
Estimated fair value	908,054
Corporate debt securities
Available-for-sale securities
Amortized cost	534,902	$ 450,295
Gross unrealized gains	16,904	6,289
Gross unrealized losses	3,929	1,704
Estimated fair value	547,877	454,880
Residential mortgage-backed securities
Available-for-sale securities
Amortized cost	120,268	124,337
Gross unrealized gains	3,470	1,056
Gross unrealized losses	12	50
Estimated fair value	123,726	125,343
Commercial mortgage-backed securities
Available-for-sale securities
Amortized cost	109,187	67,210
Gross unrealized gains	4,420	479
Gross unrealized losses	393	148
Estimated fair value	113,214	67,541
Collateralized debt obligations
Available-for-sale securities
Amortized cost	107,787	78,059
Gross unrealized gains	373	44
Gross unrealized losses	1,239	247
Estimated fair value	106,921	77,856
Other debt securities
Available-for-sale securities
Amortized cost	15,792	5,049
Gross unrealized gains	542	71
Gross unrealized losses	18	39
Estimated fair value	16,316	5,081
Available-for-sale securities
Available-for-sale securities
Amortized cost	887,936	724,950
Gross unrealized gains	25,709	7,939
Gross unrealized losses	5,591	2,188
Estimated fair value	$ 908,054	$ 730,701

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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