Investments - Cost and Fair Value, Net of Credit Loss Allowance, of Available-For-Sale Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Available-for-sale securities
Amortized cost	$ 915,034	$ 898,790
Gross unrealized gains	23,634	31,569
Gross unrealized losses	1,870	2,123
Estimated fair value	936,798	928,236
Corporate debt securities
Available-for-sale securities
Amortized cost	548,975	546,096
Gross unrealized gains	16,666	21,843
Gross unrealized losses	952	1,514
Estimated fair value	564,689	566,425
Collateralized debt obligations
Available-for-sale securities
Amortized cost	115,087	110,121
Gross unrealized gains	574	657
Gross unrealized losses	219	331
Estimated fair value	115,442	110,447
Commercial mortgage-backed securities
Available-for-sale securities
Amortized cost	109,277	115,346
Gross unrealized gains	3,378	5,090
Gross unrealized losses	363	235
Estimated fair value	112,292	120,201
Residential mortgage-backed securities
Available-for-sale securities
Amortized cost	92,142	108,840
Gross unrealized gains	1,811	3,373
Gross unrealized losses	301	34
Estimated fair value	93,652	112,179
Other debt securities
Available-for-sale securities
Amortized cost	28,905	18,387
Gross unrealized gains	462	606
Gross unrealized losses	35	9
Estimated fair value	29,332	$ 18,984
U.S. Treasury
Available-for-sale securities
Amortized cost	20,648
Gross unrealized gains	743
Gross unrealized losses	0
Estimated fair value	$ 21,391

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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