Investments - Cost and Fair Value, Net of Credit Loss Allowance, of Available-For-Sale Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Available-for-sale securities
Amortized cost	$ 899,942	$ 898,790
Gross unrealized gains	21,767	31,569
Gross unrealized losses	3,387	2,123
Estimated fair value	918,322	928,236
Corporate debt securities
Available-for-sale securities
Amortized cost	535,737	546,096
Gross unrealized gains	16,149	21,843
Gross unrealized losses	2,071	1,514
Estimated fair value	549,815	566,425
Residential mortgage-backed securities
Available-for-sale securities
Amortized cost	104,937	108,840
Gross unrealized gains	2,123	3,373
Gross unrealized losses	411	34
Estimated fair value	106,649	112,179
Commercial mortgage-backed securities
Available-for-sale securities
Amortized cost	112,028	115,346
Gross unrealized gains	2,413	5,090
Gross unrealized losses	476	235
Estimated fair value	113,965	120,201
Collateralized debt obligations
Available-for-sale securities
Amortized cost	114,819	110,121
Gross unrealized gains	634	657
Gross unrealized losses	246	331
Estimated fair value	115,207	110,447
Other debt securities
Available-for-sale securities
Amortized cost	21,912	18,387
Gross unrealized gains	435	606
Gross unrealized losses	56	9
Estimated fair value	22,291	$ 18,984
U.S. Treasury
Available-for-sale securities
Amortized cost	10,509
Gross unrealized gains	13
Gross unrealized losses	127
Estimated fair value	$ 10,395

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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