Investments - Cost and Fair Value of Available-For-Sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Available-for-sale securities
Amortized cost	$ 898,790	$ 724,950
Gross unrealized gains	31,569	7,939
Gross unrealized losses	2,123	2,188
Estimated fair value	928,236	730,701
Corporate debt securities
Available-for-sale securities
Amortized cost	546,096	450,295
Gross unrealized gains	21,843	6,289
Gross unrealized losses	1,514	1,704
Estimated fair value	566,425	454,880
Residential mortgage-backed securities
Available-for-sale securities
Amortized cost	108,840	124,337
Gross unrealized gains	3,373	1,056
Gross unrealized losses	34	50
Estimated fair value	112,179	125,343
Commercial mortgage-backed securities
Available-for-sale securities
Amortized cost	115,346	67,210
Gross unrealized gains	5,090	479
Gross unrealized losses	235	148
Estimated fair value	120,201	67,541
Collateralized debt obligations
Available-for-sale securities
Amortized cost	110,121	78,059
Gross unrealized gains	657	44
Gross unrealized losses	331	247
Estimated fair value	110,447	77,856
Other debt securities
Available-for-sale securities
Amortized cost	18,387	5,049
Gross unrealized gains	606	71
Gross unrealized losses	9	39
Estimated fair value	$ 18,984	$ 5,081

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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Amount, before tax, of unrealized loss 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).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Amount 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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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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