Investments - Summary of Available For Sale Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Amortized Cost
Amortized Cost	$ 4,404,690	$ 3,282,585
Fair Value
Fair Value	4,565,373	3,350,747
U.S. Treasury securities
Amortized Cost
Due in 1 year	78,956
Due after 1 but within 5 years	130,312
Due after 5 but within 10 years	49,125
Due after 10 years	985
Subtotal	259,378
Amortized Cost	259,378	239,087
Fair Value
Due in 1 year	79,335
Due after 1 but within 5 years	134,802
Due after 5 but within 10 years	53,165
Due after 10 years	1,142
Subtotal	268,444
Fair Value	268,444	242,206
U.S. agency securities
Amortized Cost
Due in 1 year	8,906
Due after 1 but within 5 years	9,024
Subtotal	17,930
Amortized Cost	17,930	33,620
Fair Value
Due in 1 year	8,958
Due after 1 but within 5 years	9,127
Subtotal	18,085
Fair Value	18,085	33,605
Municipal debt securities
Amortized Cost
Due in 1 year	6,881
Due after 1 but within 5 years	91,122
Due after 5 but within 10 years	235,030
Due after 10 years	180,837
Subtotal	513,870
Amortized Cost	513,870	339,511
Fair Value
Due in 1 year	6,892
Due after 1 but within 5 years	95,567
Due after 5 but within 10 years	254,387
Due after 10 years	194,671
Subtotal	551,517
Fair Value	551,517	361,638
Non-U.S. government securities
Amortized Cost
Due in 1 year	0
Due after 1 but within 5 years	22,710
Due after 5 but within 10 years	24,246
Due after 10 years	9,089
Subtotal	56,045
Amortized Cost	56,045	52,230
Fair Value
Due in 1 year	0
Due after 1 but within 5 years	24,257
Due after 5 but within 10 years	27,680
Due after 10 years	9,670
Subtotal	61,607
Fair Value	61,607	54,995
Corporate debt securities
Amortized Cost
Due in 1 year	148,292
Due after 1 but within 5 years	576,987
Due after 5 but within 10 years	322,649
Due after 10 years	22,099
Subtotal	1,070,027
Amortized Cost	1,070,027	856,638
Fair Value
Due in 1 year	149,617
Due after 1 but within 5 years	602,286
Due after 5 but within 10 years	350,632
Due after 10 years	23,977
Subtotal	1,126,512
Fair Value	1,126,512	880,301
U.S. agency mortgage-backed securities
Amortized Cost
Amortized Cost	963,670	836,710
Fair Value
Fair Value	995,905	848,334
Residential and commercial mortgage securities
Amortized Cost
Amortized Cost	391,921	282,840
Fair Value
Fair Value	409,282	288,281
Asset-backed securities
Amortized Cost
Amortized Cost	452,527	326,589
Fair Value
Fair Value	454,717	326,025
Money market funds
Amortized Cost
Amortized Cost	679,322	315,360
Fair Value
Fair Value	$ 679,304	$ 315,362

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82898722-210454

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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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