Investments - Summary of Available for Sale (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 4,404,690	$ 3,282,585
Unrealized Gains	164,423	74,231
Unrealized Losses	(3,740)	(6,069)
Fair Value	4,565,373	3,350,747
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	259,378	239,087
Unrealized Gains	9,088	3,526
Unrealized Losses	(22)	(407)
Fair Value	268,444	242,206
U.S. agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	17,930	33,620
Unrealized Gains	155	36
Unrealized Losses	0	(51)
Fair Value	18,085	33,605
U.S. agency mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	963,670	836,710
Unrealized Gains	33,205	13,956
Unrealized Losses	(970)	(2,332)
Fair Value	995,905	848,334
Municipal debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	513,870	339,511
Unrealized Gains	37,662	22,245
Unrealized Losses	(15)	(118)
Fair Value	551,517	361,638
Non-U.S. government securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	56,045	52,230
Unrealized Gains	5,562	2,812
Unrealized Losses	0	(47)
Fair Value	61,607	54,995
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,070,027	856,638
Unrealized Gains	56,864	24,255
Unrealized Losses	(379)	(592)
Fair Value	1,126,512	880,301
Residential and commercial mortgage securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	391,921	282,840
Unrealized Gains	18,641	6,542
Unrealized Losses	(1,280)	(1,101)
Fair Value	409,282	288,281
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	452,527	326,589
Unrealized Gains	3,246	857
Unrealized Losses	(1,056)	(1,421)
Fair Value	454,717	326,025
Money market funds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	679,322	315,360
Unrealized Gains	0	2
Unrealized Losses	(18)	0
Fair Value	$ 679,304	$ 315,362

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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-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
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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).

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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 net income (trading) and 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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