Investments - Cost and Fair Value, Net of Credit Loss Allowance, of Available-For-Sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Available-for-sale securities
Amortized cost	$ 920,399	$ 898,790
Gross unrealized gains	19,700	31,569
Gross unrealized losses	2,595	2,123
Estimated fair value	937,504	928,236
Corporate debt securities
Available-for-sale securities
Amortized cost	554,572	546,096
Gross unrealized gains	14,537	21,843
Gross unrealized losses	1,143	1,514
Estimated fair value	567,966	566,425
Collateralized debt obligations
Available-for-sale securities
Amortized cost	113,701	110,121
Gross unrealized gains	538	657
Gross unrealized losses	235	331
Estimated fair value	114,004	110,447
Commercial mortgage-backed securities
Available-for-sale securities
Amortized cost	94,972	115,346
Gross unrealized gains	2,306	5,090
Gross unrealized losses	451	235
Estimated fair value	96,827	120,201
Residential mortgage-backed securities
Available-for-sale securities
Amortized cost	126,276	108,840
Gross unrealized gains	1,660	3,373
Gross unrealized losses	705	34
Estimated fair value	127,231	112,179
Other debt securities
Available-for-sale securities
Amortized cost	25,399	18,387
Gross unrealized gains	345	606
Gross unrealized losses	17	9
Estimated fair value	25,727	$ 18,984
U.S. Treasury
Available-for-sale securities
Amortized cost	5,479
Gross unrealized gains	314
Gross unrealized losses	44
Estimated fair value	$ 5,749

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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