Investments - Cost and Fair Value of Available-For-Sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Available-for-sale securities
Amortized cost	$ 938,279
Estimated fair value	946,085
Corporate debt securities
Available-for-sale securities
Amortized cost	565,997	$ 546,096
Gross unrealized gains	9,663	21,843
Gross unrealized losses	2,495	1,514
Estimated fair value	573,165	566,425
Collateralized debt obligations
Available-for-sale securities
Amortized cost	115,344	110,121
Gross unrealized gains	456	657
Gross unrealized losses	338	331
Estimated fair value	115,462	110,447
Commercial mortgage-backed securities
Available-for-sale securities
Amortized cost	88,636	115,346
Gross unrealized gains	1,465	5,090
Gross unrealized losses	777	235
Estimated fair value	89,324	120,201
Residential mortgage-backed securities
Available-for-sale securities
Amortized cost	140,217	108,840
Gross unrealized gains	1,007	3,373
Gross unrealized losses	1,302	34
Estimated fair value	139,922	112,179
Other debt securities
Available-for-sale securities
Amortized cost	23,859	18,387
Gross unrealized gains	197	606
Gross unrealized losses	136	9
Estimated fair value	23,920	18,984
U.S. Treasury
Available-for-sale securities
Amortized cost	4,226
Gross unrealized gains	73
Gross unrealized losses	7
Estimated fair value	4,292
Available-for-sale securities
Available-for-sale securities
Amortized cost	938,279	898,790
Gross unrealized gains	12,861	31,569
Gross unrealized losses	5,055	2,123
Estimated fair value	$ 946,085	$ 928,236

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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